JP7704520B2 - Nonwovens, short cut thermoplastic fibers and filters - Google Patents

Description

The present invention relates to nonwoven fabrics, short-cut heat-bondable fibers, and filters using them.

Various types of filters made of nonwoven fabric have been proposed so far. For example, a filter medium made of a multilayered airlaid nonwoven fabric with a gradient in fiber fineness (see, for example, Patent Document 1), and a filter medium in which ultrafine fibers obtained by electrospinning are laminated on the surface of a general-purpose nonwoven fabric (see, for example, Patent Documents 2 and 3) have been proposed.

Of these, airlaid multi-layer filter media with a gradient in fiber fineness achieve low pressure loss and long filter life, but are insufficient for capturing extremely fine dust. Also, filter media in which ultrafine fibers are layered on the surface of a general-purpose nonwoven fabric have problems such as an increase in pressure loss due to the ultrafine fibers forming a surface coating, and poor adhesion to the nonwoven fabric base material, which means the fibers tend to fall off.

Patent Document 4 proposes a filter medium made of a wet-laid nonwoven fabric containing short-cut ultrafine fibers with a fiber diameter of 100 to 1000 nm (0.1 to 1 μm) and binder fibers with a single fiber fineness of 0.1 dtex or more. Although the filter medium has a better collection efficiency than conventional filters, there is a limit to how fine the binder fibers can be, so it is not sufficient to further improve the collection efficiency for carbon particles such as those contained in automobile exhaust gases.

Patent Document 5 proposes a filter medium using short-cut ultrafine fibers, characterized in that it has a multi-layer structure of two or more layers and that there is no boundary surface between two adjacent layers of the multi-layer structure. This has an excellent balance of high collection efficiency, low pressure loss, and long filter life, but as with the examples in Patent Document 4, only binder fibers with a single fiber fineness of 1.1 dtex are exemplified, leaving room for improvement in the collection efficiency for nano-sized fine particles such as carbon in exhaust gas.

JP 2004-301121 A JP 2006-289209 A JP 2007-170224 A International Publication No. 2008/130019 JP 2013-126626 A

According to the inventors' investigations, an important factor for extending the practical filter life is that the filter material is minimally deformed in response to external forces exerted on it by media or particles passing through it.
An object of the present invention is to provide a nonwoven fabric suitable as a filter medium which has a high collection efficiency, low pressure loss, is less prone to deformation when pressurized, and has a long practical life.

The present invention provides a nonwoven fabric comprising short-cut ultrafine fibers having a fiber diameter of 0.1 to 1 μm and an aspect ratio of 100 to 3000, and short-cut thermally adhesive fibers having a fiber diameter of 0.5 to 4 μm and an aspect ratio of 100 to 3000, the nonwoven fabric having a compressibility of 30% or less when pressurized with 30 kPa.
The present invention also relates to short-cut heat-adhesive fibers having a fiber diameter of 0.5 to 4 μm and an aspect ratio of 100 to 3000, and the heat-adhesive component is composed solely of polyester having a melting point of 250° C. or lower.

The present invention provides a nonwoven fabric that has a high capture rate, low pressure loss, little deformation when pressurized, and is suitable as a filter medium with a long practical life.

FIG. 1 is a cross-sectional view illustrating a portion of an example of a spinning nozzle for islands-in-the-sea type composite fibers used to produce short-cut ultrafine fibers that are the material of the present invention.

The following describes in detail the embodiments of the present invention.

[Short cut ultra-fine fibers]
The short-cut ultrafine fibers are made of a fiber-forming thermoplastic polymer. The short-cut ultrafine fibers have a fiber diameter (D) of 0.1 to 1 μm, preferably 0.2 to 0.9 μm, and particularly preferably 0.3 to 0.8 μm. If the fiber diameter (D) exceeds 1 μm, the pores appearing on the surface of the wetlaid nonwoven fabric produced by the papermaking method may have non-uniform pore diameters. On the other hand, if the fiber diameter (D) is less than 0.1 μm, the short-cut ultrafine fibers may easily fall off the mesh when the web is made by the papermaking method.

The aspect ratio of the short-cut ultrafine fibers (ratio of fiber length (L) to fiber diameter (D): L/D) is 100 to 3000, preferably 300 to 2500, and particularly preferably 500 to 2000. If the aspect ratio exceeds 3000, the fibers will become entangled in water before the web is made by the papermaking method, resulting in poor dispersion, and the pore sizes of the holes that appear on the surface of the nonwoven fabric produced by the papermaking method may be non-uniform. On the other hand, if the aspect ratio is less than 100, the entanglement between the fibers will be extremely weak, making it difficult to transition from the wire part to the blanket after the web is formed, and there is a risk of reducing process stability.

From the viewpoint of making the structure of the wet-laid nonwoven fabric uniform when the nonwoven fabric is produced by the papermaking method, reducing the occurrence of defects, and improving the yield and manufacturing efficiency when the nonwoven fabric is produced by the papermaking method, the fiber length of the short-cut ultrafine fibers is preferably 0.05 to 3 mm. Examples of methods for obtaining short-cut ultrafine fibers cut to a specified length include a method of cutting ultrafine fibers using a guillotine cutter type fiber bundle cutting device or a rotary cutter such as an Eastman type in which multiple cutter blades are arranged radially at equal intervals facing outward.

The cross section of the short cut ultrafine fibers may be a round cross section or a modified cross section.
As the polymer constituting the short cut ultrafine fibers, a thermoplastic polymer can be preferably used, for example, a polyamide-based, polyester-based, polyolefin-based, acrylate-based, or polyphenylene sulfide-based polymer. Polyester-based polymers are preferred because they are relatively easy to make ultrafine and mold, and have excellent strength and dimensional stability.

Preferred polyester polymers are polyethylene terephthalate, polytrimethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, and copolymers having these as main repeating units with aromatic dicarboxylic acids such as isophthalic acid and 5-sulfoisophthalic acid metal salt, aliphatic dicarboxylic acids such as adipic acid, sebacic acid, and succinic acid, hydroxycarboxylic acid condensates such as ε-caprolactone, and glycol components such as diethylene glycol, trimethylene glycol, tetramethylene glycol, and hexamethylene glycol, polylactic acid, polycaprolactone, polyhydroxybutyrate, polyethylene succinate, polybutylene succinate, and polybutylene succinate adipate.

Of the polyamide-based polymers, aliphatic polyamides such as polyamide 6, polyamide 66, and polyamide 610 are preferred.

Examples of polyolefin polymers include high density polyethylene, medium density polyethylene, high pressure low density polyethylene, linear low density polyethylene, isotactic polypropylene, ethylene propylene copolymer, ethylene copolymer of vinyl monomer such as maleic anhydride, atactic polystyrene, isotactic polystyrene, and syndiotactic polystyrene. These are preferred because they are not easily affected by acids or alkalis and have relatively low melting points, allowing them to be used as binder components after extraction into ultrafine fibers.

Examples of acrylate polymers include polymethyl acrylate, polyethyl acrylate, sodium polyacrylate, polymethyl methacrylate, and acrylamide cross-linked copolymers.

[Method for producing short cut ultrafine fibers]
The short cut ultrafine fibers can be produced by a conventional method, for example, the method disclosed in WO 2005/095686. In the present invention, it is preferable to use the fibers produced by this method. A preferred method for producing the short cut ultrafine fibers is described below.

From the viewpoint of obtaining a desired fiber diameter and uniformity thereof, the short-cut ultrafine fibers are preferably prepared by bundling islands-in-sea composite fibers having island components with island diameters (D) of 100 to 1000 nm (i.e., 0.1 to 1 μm) made of a fiber-forming thermoplastic polymer constituting the short-cut ultrafine fibers, and a sea component made of a polymer that is more soluble in an alkaline aqueous solution than the fiber-forming thermoplastic polymer (hereinafter, also referred to as an "alkaline aqueous solution-soluble polymer"), cutting the fibers into fiber bundles (tows), and then subjecting the fibers to an alkaline weight reduction process to dissolve and remove the sea component.

Here, the alkaline aqueous solution refers to an aqueous solution of an alkali metal such as potassium hydroxide or sodium hydroxide.
The island diameter can be measured by photographing the cross section of the islands-in-sea type composite fiber with a transmission electron microscope. When the island has a modified cross section other than a circular cross section, the island diameter (D) is the diameter of the circumscribing circle.

The dissolution rate ratio of the alkaline aqueous solution-soluble polymer forming the sea component to the fiber-forming thermoplastic polymer forming the island components is preferably 200 or more, more preferably 300 to 3000. This range is preferable because it provides good island separation. If the dissolution rate is less than 200 times, while the sea component in the center of the cross section of the islands-in-sea composite fiber is being dissolved, the island components in the surface layer of the cross section of the separated islands-in-sea composite fiber are dissolved due to their small diameter. Therefore, even though the amount of the sea equivalent is reduced, the sea component in the center of the cross section of the islands-in-sea composite fiber cannot be completely dissolved and removed, which leads to uneven thickness of the island components and solvent erosion of the island components themselves, and it is not preferable because it may not be possible to obtain short-cut ultrafine fibers with a uniform fiber diameter.

As the polymer that is easily soluble in an aqueous alkali solution that forms the sea part of the islands-in-sea type composite fiber, from the viewpoint of fiber formability, polyester-based polymers, aliphatic polyamides among polyamide-based polymers, and polyethylene and polystyrene among polyolefin-based polymers are preferred, and polyester-based polymers such as polylactic acid, ultrahigh molecular weight polyalkylene oxide condensation polymers, and copolymerized polyesters of polyalkylene glycol-based compounds and 5-sodium sulfoisophthalic acid are particularly preferred.

Among polyester-based polymers, a polyethylene terephthalate-based copolymer polyester having an intrinsic viscosity of 0.4 to 0.6 is preferred, which is a copolymer of 6 to 12 mol % of 5-sodium sulfoisophthalic acid and 3 to 10 mass % of polyethylene glycol having a molecular weight of 4,000 to 12,000. Here, 5-sodium sulfoisophthalic acid contributes to improving hydrophilicity and melt viscosity, and polyethylene glycol (PEG) improves hydrophilicity. In addition, the larger the molecular weight of PEG, the more hydrophilic it is thought to be due to its higher-order structure, but since its reactivity becomes poor and it becomes a blend system, problems may occur in terms of heat resistance and spinning stability. In addition, if the amount of PEG copolymerized exceeds 10 mass %, it is not preferable because it has a melt viscosity reducing effect.

In the islands-in-sea composite fiber, it is preferable that the melt viscosity of the sea component during melt spinning is greater than the melt viscosity of the island component polymer. When this relationship is met, even if the composite mass ratio of the sea component is reduced to less than 40%, there is little bonding between the islands or bonding of most of the island components, and the fiber is likely to be an islands-in-sea composite fiber.

The melt viscosity ratio (sea/island) in the islands-in-sea composite fiber is preferably 1.1 to 2.0, and particularly preferably 1.1 to 1.5. If this ratio is less than 1.1, the island components tend to bond during melt spinning, which is undesirable. On the other hand, if it exceeds 2.0, the viscosity difference is too large, which tends to reduce the spinning condition, which is undesirable.

The short cut ultrafine fibers are derived from the islands of the islands-in-sea composite fibers, and the number of islands in the cross section of the islands-in-sea composite fibers is preferably 100 or more, and more preferably 300 to 1000. The sea-island composite mass ratio (sea:island), which is the mass ratio of the sea component to the island component, is preferably 5:95 to 95:5. This range is preferable because it is possible to reduce the thickness of the sea component between the island component and the other adjacent island components, making it easier to dissolve and remove the sea component and facilitating the conversion of the island components to ultrafine fibers. If the proportion of the sea component exceeds 95%, the thickness of the sea component becomes too thick, while if it is less than 5%, the amount of the sea component is too small and bonds are more likely to occur between the islands.

Any type of spinneret can be used for melt spinning, including those with hollow pins or fine holes to form island components. For example, a spinneret may be used in which island components extruded from hollow pins or fine holes are joined with a sea component flow, the flow path of which is designed to fill the gaps between them, and the resulting mixture is compressed to form an island-sea cross section. An example of a spinneret with hollow pins is shown in Figure 1.

In the spinneret 1 shown in Figure 1, the molten island component polymer in the island component polymer reservoir 2 before distribution is distributed into the island component polymer introduction passage 3 formed by a plurality of hollow pins, while the molten sea component polymer is introduced into the sea component polymer reservoir 5 before distribution through the sea component polymer introduction passage 4. The hollow pins forming the island component polymer introduction passage 3 each penetrate the sea component polymer reservoir 5 and open downward at the center of the entrance of each of a plurality of core-sheath type composite flow passages 6 formed below. From the lower end of the island component polymer introduction passage 3, the island component polymer flow is introduced into the center of the core-sheath composite flow passage 6, and the sea component polymer flow in the sea component polymer reservoir 5 is introduced into the core-sheath composite flow passage 6 so as to surround the island component polymer, forming a core-sheath composite flow with the island component polymer flow as the core and the sea component polymer flow as the sheath, and the multiple core-sheath composite flows are introduced into a funnel-shaped confluence passage 7, where the sheaths of the multiple core-sheath composite flows come close to each other to form an islands-in-sea composite flow. While flowing down the funnel-shaped confluence passage 7, the horizontal cross-sectional area of the islands-in-sea composite flow gradually decreases, and it is discharged from the discharge port 8 at the lower end of the confluence passage 7.

The extruded islands-in-the-sea composite fiber is solidified by cooling air and taken up by a rotating roller or ejector set at a predetermined take-up speed to obtain an undrawn yarn. This take-up speed is preferably 200 to 5000 m/min. A take-up speed of less than 200 m/min is undesirable because of poor productivity. On the other hand, a take-up speed of more than 5000 m/min is undesirable because of poor spinning stability.

Depending on the use and purpose of the ultrafine fibers obtained after the extraction and removal of the sea component, the obtained undrawn yarn may be directly subjected to a cutting process, or may be subjected to a drawing process or a heat treatment process followed by a cutting process to obtain the desired strength, elongation, and heat shrinkage properties. The drawing process may be a separate drawing process in which spinning and drawing are performed in separate steps, or a direct drawing process in which drawing is performed immediately after spinning in a single process.

Island-in-sea composite fibers are cut so that the aspect ratio (ratio of fiber length (L) to island diameter (D): L/D) of the short-cut ultrafine fibers made of island component polymers is within the range of 100 to 3000. This cutting can be done by cutting the undrawn or drawn yarn as is, or by bundling it into a tow of tens to millions of yarns, with a guillotine cutter or rotary cutter. It may also be cut after the sea component extraction and removal process. The sea component extraction and removal process is carried out by alkaline weight reduction processing.

In this sea component extraction and removal process, the ratio (bath ratio) of the islands-in-the-sea composite fiber to the alkaline aqueous solution is, for example, 0.1 to 5% by mass, and preferably 0.4 to 3% by mass. If it is less than 0.1% by mass, there is a high level of contact between the fiber and the alkaline solution, but the processability of drainage, etc. may become difficult. On the other hand, if it exceeds 5% by mass, the amount of fiber is too high, and there is a risk of entanglement of the fibers during the sea component extraction and removal process.

The liquor ratio is defined by the following formula.
Bath ratio (mass%) = [mass of sea-island composite fiber (g)/mass of alkaline aqueous solution (g)] x 100

The treatment time of the alkali weight reduction process in the sea component extraction and removal step is, for example, 5 to 60 minutes, preferably 10 to 30 minutes. If it is less than 5 minutes, the alkali weight reduction may be insufficient. On the other hand, if it exceeds 60 minutes, the island component may also be reduced in weight.
The treatment temperature for the alkali weight reduction process in the sea component extraction and removal step is, for example, 50 to 90°C, preferably 60 to 80°C.

In the alkaline weight reduction process in the sea component extraction and removal step, the alkaline concentration of the alkaline aqueous solution is preferably 2 to 10% by mass. If it is less than 2% by mass, there will be a shortage of alkali, and the weight reduction rate may become extremely slow, which is not preferred. On the other hand, if it exceeds 10% by mass, the alkaline weight reduction will proceed too much, and there is a risk that even the island parts will be reduced in weight, which is not preferred.

As a method of alkaline weight reduction in the sea component extraction and removal process, a method can be used in which the islands-in-the-sea composite fiber is placed in an alkaline solution, treated under specified conditions for a specified time, and then subjected to a dehydration process once, after which it is placed again in water and neutralized and diluted using organic acids such as acetic acid and oxalic acid, before finally being dehydrated.

Alternatively, a method can be used in which neutralization is performed after a predetermined treatment time, water is further injected to proceed dilution, and then dehydration is performed. In the former case, since the treatment is performed in a batch manner, small-scale production (processing) can be performed, but the neutralization treatment takes time, resulting in slightly poor productivity. In the latter case, semi-continuous production is possible, but a large amount of acid-based aqueous solution is required during neutralization and a large amount of water is required for dilution.

In order to prevent fiber loss during dehydration, it is preferable to use a mesh-like material (such as a non-alkali hydrolyzable bag) with an opening ratio (area of openings per unit area) of 10 to 50% as disclosed in Japanese Patent No. 3678511. In this case, an opening ratio of less than 10% is undesirable as it will result in extremely poor water escape, while an opening ratio of more than 50% is undesirable as it may cause ultrafine fibers to fall off.

After the sea component extraction and removal process, it is preferable to attach a dispersant (e.g., Meikasurf manufactured by Meisei Chemical Industry Co., Ltd.) to the surface of the short-cut ultrafine fibers in an amount of 0.1 to 5.0 mass % based on the mass of the fibers in order to increase the dispersibility of the short-cut ultrafine fibers.

By subjecting the islands-in-sea composite fiber to the above-described sea component extraction and removal process, which is a short-cut fiber obtained by cutting the islands-in-sea composite fiber to a predetermined length, the islands-in-sea composite fiber is converted into the short-cut ultrafine fiber of the present invention, which is made of island components. In this case, if the island components are made of a polyester-based polymer, it is possible to obtain short-cut ultrafine fibers made of polyester.

[Short cut heat-bondable fiber]
In the present invention, the short-cut heat-bondable fibers are used as binder fibers for maintaining the short-cut ultrafine fibers in the form of a nonwoven fabric. By using the short-cut heat-bondable fibers, the efficiency of collecting fine particles can be significantly improved when the nonwoven fabric is used as a filter medium.

The fiber diameter (D) of the short-cut heat-adhesive fiber is 0.5 to 4 μm, preferably 0.8 to 3 μm, and more preferably 1.0 to 2.8 μm. If the fiber diameter (D) is less than 0.5 μm, the rigidity of the fiber itself may be reduced and the structure of the filter medium may not be easily maintained, which is undesirable. On the other hand, if it exceeds 4 μm, the number of short-cut heat-adhesive fibers in the filter medium will be reduced, the number of bonding points will decrease, the rigidity will be reduced, the pore size of the filter medium will increase, and the efficiency of collecting fine particles may decrease, which is undesirable.

The aspect ratio of the short-cut heat-bondable fibers (ratio of fiber length (L) to single fiber diameter (D): L/D) is 100 to 3000, preferably 300 to 2500, and particularly preferably 500 to 2000. If the aspect ratio is less than 100, the entanglement between the fibers will be extremely weak, and the transition from the wire part to the blanket after the web is formed may be difficult, resulting in a decrease in process stability. On the other hand, if the aspect ratio exceeds 3000, the fibers may entangle with each other in water before the web is made up in the papermaking process, resulting in poor dispersion, and the pore size of the holes that appear on the surface of the wet nonwoven fabric may be nonuniform (i.e., the ratio of the average pore size to the maximum pore size may be large).

From the viewpoints of uniformity of the structure of the wet-laid nonwoven fabric, minimal occurrence of defects, and problems in the production of the wet-laid nonwoven fabric, yield, and production efficiency, the fiber length of the short-cut heat-adhesive fibers is preferably 0.05 to 10 mm.

Methods for obtaining short-cut heat-bondable fibers of a specified length include a guillotine cutter type fiber bundle cutting device and a rotary cutter such as an Eastman type with multiple cutter blades facing outward and spaced radially at equal intervals.

One preferred embodiment of the short-cut thermally adhesive fiber is an embodiment in which the fiber-forming component accounts for 20 to 80% by mass and the thermally adhesive component accounts for 80 to 20% by mass,
In this embodiment, the fiber-forming component is a polyester having a melting point of 180° C. or higher, and the thermal adhesive component is a polyester having a melting point that is at least 20° C. lower than the melting point of the fiber-forming component.

If the short-cut thermal adhesive fiber is less than 20% by weight, the nonwoven fabric will not be able to maintain its shape under a certain pressure, which is undesirable. On the other hand, if it exceeds 80% by weight, the flowable portion of the thermal adhesive component increases, reducing the spatial volume for capturing the collected matter, which undesirably reduces the collection efficiency and filter life.

The fiber-forming component in the short-cut heat-bondable fiber is a polyester having a melting point of 180°C or higher, preferably 190 to 270°C, with alkylene terephthalate as the main repeating component. If the melting point of this polyalkylene terephthalate polyester is less than 180°C, not only will it be difficult to stably spin the composite fiber, but the dimensional stability during the heat-bonding process will also decrease, which is undesirable. On the other hand, if the melting point exceeds 270°C, it will be difficult to make the fiber extremely thin, which is undesirable.

As the polyalkylene terephthalate polyester of the fiber-forming component, polyethylene terephthalate, polytrimethylene terephthalate, and polybutylene terephthalate are preferred, with polyethylene terephthalate being particularly preferred because it is inexpensive and versatile. In order to improve the fiber moldability for ultra-thinning, monomer components such as isophthalic acid, 5-sulfoisophthalic acid metal salt, 5-sulfoisophthalic acid organic phosphorus salt, 2,6-dinaphthalene acid, adipic acid, sebacic acid, azelaic acid, dodecanoic acid, ε-caprolactone, 1,3-cyclohexanedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, diethylene glycol, polyalkylene glycol, 1,3-cyclohexanedimethanol, 1,4-cyclohexanedimethanol, trimethylene glycol, tetramethylene glycol, hexamethylene glycol, triethylene glycol, and tetraethylene glycol may be copolymerized to a range in which the melting point falls within a predetermined range.

The thermal adhesive component in the short-cut thermally adhesive fiber is preferably a polyester having a melting point 20°C or more lower than that of the fiber-forming component. In this case, the polyester is preferably a polyalkylene terephthalate polyester having a melting point of 250°C or less. By having the melting point of the thermal adhesive component 20°C or more lower than that of the fiber-forming component, good adhesive strength with the short-cut ultrafine fiber can be obtained. And, by having the thermal adhesive component be a polyalkylene terephthalate polyester having a melting point of 250°C or less, good adhesive strength and elasticity and rigidity can be obtained that make it difficult for the space in the nonwoven fabric to collapse even under pressure.

The polyalkylene terephthalate polyester of the thermal adhesive component is preferably a polyalkylene polyester having ethylene terephthalate, trimethylene terephthalate, butylene terephthalate, or hexamethylene terephthalate as the main repeating unit, and may be copolymerized with dicarboxylic acids such as isophthalic acid, 5-sulfoisophthalic acid metal salt, 5-sulfoisophthalic acid organic phosphorus salt, 2,6-dinaphthalene acid, adipic acid, sebacic acid, azelaic acid, dodecanoic acid, ε-caprolactone, 1,3-cyclohexanedicarboxylic acid, and 1,4-cyclohexanedicarboxylic acid, diethylene glycol, polyalkylene glycol, 1,3-cyclohexanedimethanol, 1,4-cyclohexanedimethanol, trimethylene glycol, tetramethylene glycol, hexamethylene glycol, triethylene glycol, and tetraethylene glycol, in order to adjust the thermal adhesive temperature, adjust the bonding temperature and pressure, and improve fiber moldability for ultra-thinning.

The above-mentioned composite fibers need to be composited so that the thermal adhesive component is exposed on the surface of the composite fiber in order to thermally bond the composite fibers. Examples of this embodiment include those in which the thermal adhesive component and the fiber-forming component are composited in a parallel (side-by-side) type, concentric core-sheath type composite fibers in which the thermal adhesive component is composited as the sheath component and the fiber-forming component is composited as the core component, and eccentric core-sheath type composite fibers, among which concentric core-sheath type composite fibers are preferred. Concentric core-sheath type composite fibers can obtain good bonding points between short-cut ultrafine fibers or between short-cut thermal adhesive fibers, and the fiber-to-fiber distance that affects the pore size, which is important for filter performance, is uniform, and are also good in terms of dimensional stability under high temperatures and high pressures. It should be noted that even if the concentric core-sheath type composite fibers are called concentric core-sheath type composite fibers, there is no problem if the center of gravity of the core and the center of gravity of the sheath are shifted within an acceptable range, and they are considered to be concentric core-sheath type composite fibers unless they are intentionally eccentric.

As another form of the short-cut heat-bondable fiber, a fiber may be used in which the heat-bonding component of the short-cut heat-bondable fiber is a single component of polyester having a melting point of 250° C. or less. In this case, by heat-sealing or pressing the short-cut heat-bondable fibers together or the short-cut ultrafine fibers, the pore size can be made smaller than that of conventionally known heat-bondable fibers, in combination with the fineness of the short-cut heat-bondable fiber of the present invention. To achieve this, it is preferable that the melting point of the short-cut heat-bondable fiber is 5° C. or more lower than the melting point of the short-cut ultrafine fiber.
The short cut thermally adhesive fibers may have any cross-sectional shape, for example round, hollow, cross, flat, or finned.

The above-mentioned polymers may also contain various stabilizers, UV absorbers, thickening and branching agents, viscosity reducers, matting agents, colorants, antibacterial agents, deodorants, carbon black, hydrophilic agents, water repellent agents, metal particles, metal oxides, antioxidants, light stabilizers, fluorescent whitening agents, and various other improving agents, as necessary.

[Method for producing short cut heat-bondable fibers]
The extremely fine short-cut thermal adhesive fibers having a fiber diameter of 0.5 to 4 μm in the present invention can be produced, for example, by the following method.

When the short-cut thermally adhesive fiber is a composite fiber consisting of a fiber-forming component and a thermally adhesive component, the polymers constituting the thermally adhesive component and the fiber-forming component described above are formed into chips, which are then dried in a heated or dehumidified nitrogen or air atmosphere, or in a vacuum, melted, and introduced into a composite spinneret to be extruded as a molten composite fiber yarn, which is cooled and solidified at a position 15 to 100 mm below the spinneret, and taken up at a spinning speed of 300 to 1000 m/min to obtain an undrawn yarn. A known composite spinneret can be used.

When the thermal adhesive component of the short cut thermal adhesive fiber is a single component of polyalkylene terephthalate polyester with a melting point of 250°C or less, the polymer that constitutes the thermal adhesive component is formed into chips, dried and melted in the same manner as described above, and introduced into a single component spinneret with a known single-hole nozzle, where it is cooled, solidified, and taken up to obtain an undrawn yarn.

As a method for obtaining short-cut heat-adhesive fibers with the desired fiber diameter, an undrawn yarn can be drawn at a high draw ratio of 5 times or more by flow drawing while minimizing changes to the orientation and crystallization of the amorphous parts in the undrawn yarn, and then, if necessary, the draw ratio can be further increased by neck drawing until the undrawn parts disappear, resulting in a total draw ratio of 10 times or more.

At this time, it is preferable to apply a polyether-polyester copolymer to the surface of the undrawn yarn. Flow drawing is performed in warm water that is 5 to 50°C higher than the higher of the glass transition temperatures of the thermal adhesive component and the fiber-forming component of the undrawn yarn. By applying a polyether-polyester copolymer to the surface of the undrawn yarn, a film is formed on the surface of the undrawn yarn, making it difficult for fusion and adhesion between the fibers to occur during flow drawing in high-temperature warm water.

Furthermore, when the obtained fibers are subjected to a papermaking process to obtain a wetlaid nonwoven fabric, the polyether-polyester copolymer adhered to the fibers improves the dispersibility of the fibers in water. When the polyether-polyester copolymer is applied to the undrawn yarn, it is preferable to apply it by an oiling device immediately after the undrawn yarn is spun, or to apply it by immersing it in a warm water bath in the flow drawing process.

The undrawn yarn is then flow-drawn, and if necessary, neck-drawn or subjected to limited heat shrinkage treatment in a relaxed state. After that, a fiber treatment agent is applied according to the nonwoven fabric forming process and the desired function, and the yarn is cut with a guillotine cutter or rotary cutter to obtain the desired short-cut heat-bondable fiber.

In addition, when the nonwoven fabric is a wet nonwoven fabric, applying an aqueous emulsion of polyether-polyester copolymer similar to that applied to the undrawn yarn as a fiber treatment agent after drawing or before or after cutting will further improve the dispersibility of the fibers in water in the wet web formation process. In this case, the polyether-polyester copolymer should be applied in an amount of 0.03 to 10.0% by mass per 100% by mass of the short-cut heat-bondable fibers. If it is less than 0.03% by mass, the dispersion of the fibers in water in the papermaking process will be insufficient, which is undesirable, while if it exceeds 10.0% by mass, not only will the adhesion between the fibers tend to be impaired, but a large amount of polyether-polyester copolymer will also increase the water quality load on the circulating water in the wet web formation process, which is undesirable.

The polyether-polyester copolymer is composed of terephthalic acid and/or isophthalic acid, lower alkylene glycol, and polyalkylene glycol and/or its monoether. Examples of the lower alkylene glycol that is preferably used include ethylene glycol, propylene glycol, and tetramethylene glycol. On the other hand, examples of the polyalkylene glycol include polyethylene glycol, polyethylene glycol-polypropylene glycol copolymer, and polypropylene glycol having an average molecular weight of 600 to 6000. Furthermore, examples of the monoether of the polyalkylene glycol include the monomethyl ether, monoethyl ether, and monophenyl ether of polyethylene glycol, polypropylene glycol, etc.

The above polyether-polyester copolymer preferably has a molar ratio of terephthalate units to isophthalate units in the range of 95:5 to 40:60 from the viewpoint of water dispersibility, but may also contain small amounts of alkali metal salts of sulfoisophthalic acid, adipic acid, sebacic acid, etc.

The average molecular weight of the polyether-polyester copolymer depends on the molecular weight of the polyalkylene glycol used, but is usually 1,000 to 20,000, and preferably 3,000 to 15,000. If the average molecular weight is less than 1,000, the effect of improving dispersibility in water is insufficient, while if it exceeds 20,000, it becomes difficult to emulsify and disperse the polymer.

In order to improve flow stretchability and obtain extremely fine short-cut heat-bondable fibers with a fiber diameter of 0.5 to 4 μm, the short-cut heat-bondable fibers are made of polyester, and the polyester is preferably a copolymer polyester having 60 to 90 mol % of a terephthalic acid component and 10 to 40 mol % of an isophthalic acid component as dicarboxylic acid components.

By copolymerizing polyester with 10-40 mol% isophthalic acid, the flow stretch ratio can be significantly improved. Whereas the ratio remains at 10 times or less when not copolymerized, copolymerization with isophthalic acid can improve the ratio to around 20-200 times. The reason for this is unclear, but it is assumed that the possible stretch ratio is improved because it is possible to reduce crystallinity and the orientation of amorphous molecules in undrawn yarn and fibers after flow stretching, and, although it depends on the copolymerization, the free volume (excluded volume) around the molecules during flow stretching increases even if the molecular weight is increased, making it easier for molecular chains to move at temperatures higher than the glass transition temperature, and the molecular weight can be increased to a level where sufficient entanglement of the molecular chains can be ensured.

If the copolymerization amount of the isophthalic acid component is less than 10 mol%, the flow drawing ratio decreases, and it becomes difficult to obtain short-cut heat-adhesive fibers with a fiber diameter of 4 μm or less, which is not preferable. On the other hand, if the copolymerization amount of the isophthalic acid component exceeds 40 mol%, it becomes difficult to suppress fusion between fibers during flow drawing, and it becomes difficult to produce the desired ultrafine fibers uniformly and stably, which is also not preferable. The copolymerization amount of the isophthalic acid component is more preferably 15 to 35 mol%, and particularly preferably 20 to 30 mol%, based on the total dicarboxylic acid components.

When the short cut heat-bondable fiber is in the form of a composite fiber, it is preferable that an isophthalic acid component is copolymerized in both the fiber-forming component and the heat-bonding component, and the copolymerization amount of the isophthalic acid component may differ between the fiber-forming component and the heat-bonding component, but even in this case, it is preferable that the weighted average copolymerization amount of the isophthalic acid component of both (i.e., for the entire fiber) is in the range of 10 to 40 mol%.

The copolymer polyester is composed mainly of alkylene terephthalate as a repeating component, and the repeating unit is preferably ethylene terephthalate, trimethylene terephthalate, butylene terephthalate, or hexamethylene terephthalate. As the diol component of the copolymer polyester, it is preferable to use an aliphatic diol.

Among the above copolymerized polyesters, isophthalic acid copolymerized polyalkylene terephthalate polyesters having ethylene terephthalate as the main repeating unit are most preferred. In order to adjust the flow stretchability and the melting point and adhesive strength of the thermal adhesive component, other than isophthalic acid, bifunctional carboxylic acids such as adipic acid, sebacic acid, and sodium 3-5-dicarboxybenzenesulfonate, and diols such as diethylene glycol, propylene glycol, trimethylene glycol, tetramethylene glycol, hexamethylene glycol, and polyethylene glycol may also be copolymerized.

[Nonwoven fabric]
The nonwoven fabric of the present invention contains the short-cut ultrafine fibers and short-cut thermal adhesive fibers defined above, and may contain other short-cut fibers as necessary, and can be obtained by forming a web and then molding it into the form of a nonwoven fabric by heating or pressing.

Examples of web formation methods include the carding method, the air-laid method, and the papermaking method, and it is particularly preferable to form a wet nonwoven fabric by the papermaking method. Examples of web joining methods include the air-through method, the embossing method, the Yankee dryer method, the multi-cylinder dryer method, the calendar method, the resin bond method, and the water jet method, and the calendar method and the water jet method (sometimes called the spunlace method, hydroentanglement method, or hydroentanglement method) are particularly suitable.

In the nonwoven fabric of the present invention, the mass ratio of the short-cut ultrafine fibers to the total mass of the nonwoven fabric is preferably 0.5 to 50 mass%, more preferably 1 to 40 mass%, and particularly preferably 3 to 30 mass%. If the short-cut ultrafine fibers are less than 0.5 mass%, not only will a satisfactory collection efficiency not be obtained, but the wet nonwoven fabric may develop uneven texture, which is undesirable. On the other hand, if the mass ratio exceeds 50 mass%, the filter medium (wet nonwoven fabric) becomes too dense, resulting in extremely poor filterability in the papermaking process, reduced productivity, and excessive pressure loss, which is undesirable.

In the nonwoven fabric of the present invention, the mass ratio of the short-cut thermally adhesive fibers to the total mass of the nonwoven fabric is preferably 10 to 99.5% by mass, more preferably 10 to 95.5% by mass, even more preferably 15 to 90% by mass, and particularly preferably 20 to 85% by mass. If the short-cut ultrafine fibers are less than 10% by mass, the thermal adhesiveness is low, the strength of the nonwoven fabric and the deformation under pressure are insufficient, a uniform pore size cannot be obtained, the collection efficiency and filter life are poor, and the quality is unstable. In addition, the formation of the wet nonwoven fabric may be uneven, which is unsuitable. On the other hand, if the ratio exceeds 95.5% by mass, the effect of the short-cut ultrafine fibers is insufficient, and the filtration performance (collection efficiency) is not as good as that of the conventional technology using ultrafine fibers.

The nonwoven fabric of the present invention is characterized by the use of short-cut heat-adhesive fibers with a small fiber diameter, which increases the number of bonding points of the heat-adhesive component, and by making the distance between the fibers uniform, the shape of the filter medium is maintained, it is not easily crushed by pressure, and it has high collection efficiency, low pressure loss, and a long filter life.

In the nonwoven fabric of the present invention, fibers other than the short-cut ultrafine fibers and short-cut heat-adhesive fibers may be mixed with various synthetic fibers (polyethylene terephthalate, polytrimethylene terephthalate, polybutylene terephthalate, nylon, olefin-based, aramid-based), natural pulp such as wood pulp and linter pulp, and synthetic pulp mainly composed of aramid or polyethylene, so long as they account for 50% by mass or less of the total mass of the nonwoven fabric. In particular, polyethylene terephthalate short fibers made of stretched polyethylene terephthalate with a single fiber fineness of 0.01 to 0.6 dtex and a fiber length of 1 to 10 mm are preferred from the standpoint of dimensional stability, etc.

In addition, the nonwoven fabric of the present invention may be in a form in which the short-cut ultrafine fibers and the short-cut heat-bonding fibers are mixed with two or more types of short-cut fibers having different fiber diameters, polymer types, or compositions, depending on the purpose, as long as the requirements including the short-cut ultrafine fibers and the short-cut heat-bonding fibers are satisfied. In addition, the nonwoven fabric may be a single layer with a uniform mixing ratio overall, but may be composed of two or more layers depending on the purpose. In that case, in order to use it as a filter medium, it is preferable that the mixing ratio and fiber density have a gradient structure (the mixing ratio and fiber density gradually increase and decrease) in the thickness direction of the nonwoven fabric in which there is no boundary surface between adjacent layers, and such a form can be achieved by arranging multiple carding machines, air-laid web manufacturing devices, and papermaking machines in the web formation process, laminating layers with different mixing ratios and fiber types, and then fixing the fibers between them by thermal bonding or thermocompression bonding. In addition, when composed of two or more layers, if the mixing ratio of the short-cut ultrafine fibers is high or the average fiber diameter of the entire fibers that are composed is small, the density can be high and the through-holes can be small. By creating density differences between the web layers in this way, it is possible to design a filter that has high collection efficiency, low pressure loss, and a long filter life. The density of the web layers may be designed to increase or decrease in the filtration direction, or may be in a layered state with a mixture of large and small densities, and may be freely designed according to the purpose.

Depending on the purpose, nonwoven fabrics of other materials may be laminated for the purpose of improving the tensile strength and hardness of the filter, further improving the filter life as a prefilter for coarse dust, pleat stability, adhesion of other materials, etc. For example, the other materials may be spunbond nonwoven fabric, meltblown nonwoven fabric, heatbond nonwoven fabric, needle punch nonwoven fabric, spunlace nonwoven fabric, airlaid nonwoven fabric, resin bond nonwoven fabric, stitchbond nonwoven fabric, tow spread nonwoven fabric, burst fiber nonwoven fabric, composite laminates of these nonwoven fabrics, and other woven and knitted fabrics.

The basis weight of the nonwoven fabric of the present invention is preferably 1 to 500 g/m ² , more preferably 3 to 400 g/m ² , and particularly preferably 5 to 300 g/m ^2. If the basis weight is less than 1 g/m ² , the nonwoven fabric is too thin and therefore has low strength, and is prone to unevenness during papermaking, which may cause defects or breakage when the filter is used, allowing the material to pass through. On the other hand, if the basis weight exceeds 500 g/m ² , water is less likely to escape during papermaking, which may cause problems in terms of production and cost.

The thickness of the nonwoven fabric of the present invention is preferably 0.01 to 3.0 mm, more preferably 0.5 to 3.0 mm, and particularly preferably 1.0 to 2.5 mm. A thickness of less than 0.01 mm is not preferred because the strength of the filter medium is insufficient. On the other hand, a thickness of more than 3.0 mm is not preferred because the compactness of the filter medium decreases and it becomes difficult to form pleats (zigzag folding structure) to increase the filtration area.

In the nonwoven fabric of the present invention, the average pore size of the through pores is preferably 0.1 to 10.0 μm, more preferably 0.3 to 5.0 μm, and even more preferably 0.5 to 2.5 μm. If the average pore size is less than 0.1 μm, the pressure loss of the filter medium becomes too high, and the air flow rate after filtration decreases, which is not preferable. On the other hand, if it exceeds 10.0 μm, the density of the filter medium decreases, and the collection rate of fine particles decreases, which is not preferable.

In the nonwoven fabric of the present invention, the maximum pore diameter/average pore diameter (the value obtained by dividing the maximum pore diameter by the average pore diameter) of the through pores is preferably 1.0 to 2.5, and more preferably 1.0 to 2.3. A maximum pore diameter/average pore diameter of 1.0 indicates that the through pores of the nonwoven fabric are uniform in diameter, and is most preferable. If the maximum pore diameter/average pore diameter exceeds 2.5, the density of the filter medium decreases, and the capture rate of fine particles decreases, which is not preferable. If the pore shape is not a perfect circle, the long diameter is taken as the pore diameter.

In the nonwoven fabric of the present invention having the above basis weight and thickness, its air permeability is preferably 0.5 to 10 cm ³ /cm ² /sec, more preferably 0.7 to 5 cm ³ /cm ² /sec, and particularly preferably 1 to 4 cm ³ /cm ² /sec. If the air permeability is less than 0.5 cm ³ /cm ² /sec, the pressure loss of the filter becomes too high, which is undesirable as the air flow rate after filtration through the filter decreases. On the other hand, if the air permeability exceeds 10 cm ³ /cm ² /sec, the denseness of the filter medium decreases, which is undesirable as the collection efficiency decreases.

The nonwoven fabric of the present invention has a compressibility of 30% or less, preferably 26% or less, when pressurized with 30 kPa, as calculated by the following formula. If the compressibility exceeds 30% when pressurized with 30 kPa, the nonwoven fabric as a filter medium will be crushed by the pressure when used as a filter, resulting in high density and shortened life.
Compression rate when compressed (%) = thickness of filter medium when compressed (mm) / thickness of filter medium before compression (mm)

[Method of manufacturing nonwoven fabric]
The nonwoven fabric of the present invention can be produced by making a wetlaid nonwoven fabric by multi-layer papermaking using a conventional Fourdrinier papermaking machine, short wire papermaking machine, cylinder papermaking machine, or a combination of a plurality of these machines, and then heat treating the fabric.

The heat treatment step is carried out after the web formation step by the papermaking method, and either a Yankee dryer or an air-through dryer can be used for the heat treatment. The heat treatment temperature is usually 100 to 140°C, preferably 110 to 130°C, and the heat treatment time is, for example, 30 to 300 seconds, preferably 60 to 180 seconds. The nonwoven fabric may be subjected to calendaring, embossing, or flat plate hot press processing as necessary. In the case of calendaring, the temperature of the calendar rollers is, for example, 140 to 250°C, and the linear pressure between the rollers is, for example, 1 to 100 kN/m.

[Filter]
The nonwoven fabric of the present invention uses short-cut ultrafine fibers and thermally adhesive fibers having extremely fine fiber diameters, and when used as a filter medium, a filter that satisfies all of the requirements of high collection efficiency, low pressure loss, and long life can be obtained.

Filters using the nonwoven fabric of the present invention can be suitably used as, for example, chemical filters, air filters, liquid filters, and oil filters.
The nonwoven fabric of the present invention is homogeneous and has an extremely small pore size, and therefore, in addition to filters, can be suitably used for stencil printing base paper, wipers, battery separators, artificial leather, etc.

Next, examples of the present invention and comparative examples will be described in detail. The measurement items in the examples were measured by the following methods.

(1) Intrinsic Viscosity 0.12 g of a polymer sample was dissolved in 10 mL of a mixed solvent of tetrachloroethane/phenol (volume ratio 1/1), and the intrinsic viscosity (dL/g) at 35° C. was measured.

(2) Melting point Using a Du Pont 990 differential thermal analyzer, the measurement was performed at a temperature rise rate of 20°C/min to determine the melting peak. When the melting temperature was not clearly observed, a micro melting point measuring device (Yanagimoto Seisakusho) was used to determine the temperature (softening point) at which the polymer softened and began to flow, and this was taken as the melting point. In either case, five samples were measured, and the average was calculated.

(3) Melt Viscosity A test polymer was dried and placed in an orifice of a melt spinning extruder set at the melt temperature, and after being held in a molten state for 5 minutes, it was extruded under a prescribed load, and the shear rate and melt viscosity at this time were plotted. The above operation was repeated under multiple load levels. Based on the above data, a shear rate-melt viscosity relationship curve was created. On this curve, the melt viscosity at a shear rate of 1000 sec ^-1 was estimated.

(4) Dissolution rate measurement Each of the sea and island component polymers was extruded through a spinneret for producing islands-in-sea type composite fiber having 24 nozzles with a hole diameter of 0.3 mm and a land length of 0.6 mm, and the fiber was taken up at a speed of 1000 to 2000 m/min and stretched. The elongation at break was controlled within a range of 30 to 60%, and a multifilament of 75 dtex/24 filaments was produced. This multifilament was dissolved in a solvent at a predetermined temperature with a bath ratio of 50, and the weight loss rate was calculated from the dissolution time and amount dissolved.

(5) Fiber length L
The fiber length of 100 single yarns was measured using a digital microscope (VHX-5000) manufactured by Keyence Corporation, and the average value was taken as the fiber length L (unit: mm).

(6) Fiber diameter D (in the case of short cut ultrafine fibers)
The fiber cross-section was photographed at a magnification of 30,000 times using a transmission electron microscope (TEM) (with length measurement function). The fiber diameter D was the diameter of the circumscribed circle in the cross section of a single fiber (average value of five samples).

(7) Fiber diameter D (other than short cut ultrafine fibers)
It was calculated from the single yarn fineness d (dtex) and density ρ (g/cm ³ ) by the following formula (unit: μm): where π is the circular constant.
D (μm) = 20 x (d/πρ) ^1/2

(8) Single fiber fineness (d) (other than short cut ultrafine fibers)
After drawing, 2000 mm of the fiber bundle was sampled and dried in a hot air dryer at 120° C. for 40 minutes, and the measured bone dry mass was multiplied by 5000 to measure the total fineness (unit: dtex) of the fiber bundle. The obtained total fineness was divided by the number of single filaments constituting the fiber bundle to calculate the single filament fineness d (unit: dtex).

(9) Fiber density (other than short cut ultrafine fibers)
The fiber density ρ (unit: g/cm ³ ) was measured using the density gradient tube method described in JIS L1015:2010 8.14.2.

(10) Aspect ratio (L/D)
Using the values of fiber diameter D (unit: mm) calculated from the single yarn fineness and fiber length L (unit converted from μm to mm), the ratio of fiber length (L)/fiber diameter (D) was defined as the aspect ratio (L/D).

(11) Weight per unit area Measured based on JIS P8124:2011 (paper and paperboard-method of measurement of basis weight).

(12) Thickness: Conducted based on JIS P8118:2014 (Paper and paperboard-Test methods for thickness, density and specific volume).

(13) Nonwoven fabric density was measured based on JIS P8118:2014 (Paper and paperboard-Test methods for thickness, density and specific volume).

(14) Breathability This was measured based on JIS L1913:2010 (general nonwoven fabric test method) 6.8.

(15) Tensile strength index Based on JIS P8113:2006 (Paper and paperboard-Test methods for tensile properties-Part 2: Constant speed extension method), the tensile strength index was measured in the dryer paper feed direction (MD) and the direction perpendicular thereto (CD) when the sample was prepared, and converted into tensile strength index (unit: N m/g).

(16) 30 kPa compressibility: Measured based on JIS L1913:2010 (general nonwoven fabric testing method) 6.14.

(17) Pore diameter Two circular samples with a diameter of 2.5 cm were randomly taken from the nonwoven fabric, and the average pore diameter, minimum pore diameter, and maximum pore diameter (unit: μm) were measured using a perm porometer (a pore size distribution measuring instrument manufactured by PMI Co., Ltd.). The maximum pore diameter/average pore diameter value was also calculated.

(18) Atmospheric Dust Collection Efficiency The wind speed was adjusted to 5.1 cm/sec, and the amount of atmospheric dust in front of and behind the sample was counted using a particle counter (KC-03B, manufactured by Rion Co., Ltd.). The collection efficiency was calculated from the ratio of the counts.
Airborne dust collection rate (%) = (1 - (airborne dust number after passing the sample / airborne dust number before passing the sample)) x 100

(19) Pressure Loss The pressure before and after the test piece passed when measuring the air dust collection rate (wind speed 5.1 cm/sec) was measured, and the pressure difference was calculated as the pressure loss.

(20) Filter life Eight types of JIS Z 8901 were used as test dusts, and the flow velocity when passing through the sample was 16.7 cm/sec, the dust concentration was 1 g/ ^m3 , and the amount of dust collected (mass increase) was measured (g/ ^m2 ) when the pressure drop increased by 2 kPa when the dust concentration was 1 g/m3.

[Example 1]
The island component was polyethylene terephthalate having a melt viscosity of 120 Pa sec at 285° C. (intrinsic viscosity 0.64 dL/g, melting point 256° C.), and the sea component was modified polyethylene terephthalate having a melt viscosity of 135 Pa sec at 285° C. and copolymerized with 4 mass % of polyethylene glycol having an average molecular weight of 4000 and 9 mol % of 5-sodium sulfoisophthalic acid (intrinsic viscosity 0.39 dL/g, melting point 226° C.). The mass ratio of sea:island was 10:90, and the fibers were spun using a spinneret for islands-in-sea type composite fiber having 400 islands and the structure shown in FIG. 1, and taken up at a spinning speed of 1500 m/min. The difference in alkali weight loss rate between the island component and the sea component was 1000 times.

After stretching this 3.9 times, it was cut to a fiber length of 1 mm with a guillotine cutter to obtain composite fibers for short-cut ultrafine fiber A. When this was reduced in weight by 10% at 75°C in a 4% by mass NaOH aqueous solution, ultrafine short fibers with relatively uniform fiber diameter and fiber length were produced. These were designated short-cut ultrafine fiber A. The fiber diameter of the obtained short-cut ultrafine fiber A was 0.75 μm, the fiber length was 0.8 mm, and the aspect ratio was 1067.

On the other hand, we prepared a core-sheath type composite fiber (core:sheath = 50:50 mass ratio) in which the fiber-forming component was polyethylene terephthalate copolymerized with 20 mol% isophthalic acid (intrinsic viscosity 0.64 dL/g, melting point 202°C), the thermal adhesive component was polyethylene terephthalate copolymerized with 40 mol% isophthalic acid and 4 mol% diethylene glycol (intrinsic viscosity 0.55 dL/g, melting point (softening point) 110°C), the fiber-forming component was the core, and the thermal adhesive component was the sheath.

The content of the isophthalic acid component, which is a copolymer component contained in this core-sheath type composite fiber, was 30 mol%, calculated as the sum of 10 mol% (20 mol% x 50%) contained in the fiber-forming component of the core and 20 mol% (40 mol% x 50%) contained in the thermal adhesive component of the sheath.

To obtain a core-sheath type composite fiber, the fiber-forming component and the thermal adhesive component were melted in separate vent-type twin-screw extruders, and then the fiber-forming component was used as the core component and the thermal adhesive component was used as the sheath component. The fiber-forming component was used as the core component, ... and the thermal adhesive component were used as the core:sheath mass ratio of 50:50. The fiber-forming component and the thermal adhesive component were then combined in a known core-sheath type composite spinneret with 1,336 capillaries with a hole diameter of 0.3 mm, and the melt was extruded into a filament. The nozzle temperature was 290°C, and the extrusion rate was 430 g/min.

The extruded yarn was then cooled and solidified with 25°C cooling air at a position 31 mm below the nozzle, and then wound up at 500 m/min while adding a polyether-polyester copolymer aqueous emulsion at a solids deposition rate of 0.5% by mass below the extruded yarn, to obtain an undrawn yarn.

This undrawn yarn was stretched 63 times in warm water at 82°C, and then stretched twice in warm water at 70°C. After applying a polyether-polyester copolymer aqueous solution to the fiber surface, it was cut to a fiber length of 3 mm to obtain short-cut heat-adhesive fiber B with a fineness of 0.051 dtex (fiber diameter 2.3 μm, fiber length 3 mm, aspect ratio 1328).

In addition, polyethylene terephthalate short fibers (Tepirus TA04PN SD0.1x3 manufactured by Teijin Frontier Co., Ltd. (intrinsic viscosity 0.47 dL/g, melting point 256°C, fineness 0.1 dtex, fiber diameter 4 μm, fiber length 3 mm, aspect ratio 750) were used as other fibers C.

The short-cut ultrafine fibers A, the short-cut heat-adhesive fibers B and the other fibers C were mixed and stirred in a predetermined mass ratio (short-cut ultrafine fibers A/short-cut heat-adhesive fibers B/other fibers C=15/40/45), and made into paper with a basis weight of 100 g/ ^m2 using a TAPPI (a square sheet machine manufactured by Kumagai Riki Kogyo Co., Ltd.), and then dried (120°C x 2 minutes) using a Yankee dryer to obtain a wet-laid nonwoven fabric. The physical properties of the obtained nonwoven fabric (filter material) are shown in Tables 1 and 2.

[Example 2]
A wet-laid nonwoven fabric was obtained in the same manner as in Example 1, except that the basis weight was 300 g/ ^m2 in the papermaking process of Example 1. The physical properties of the obtained nonwoven fabric (filter material) are shown in Tables 1 and 2.

[Example 3]
A wet-laid nonwoven fabric was obtained in the same manner as in Example 1, except that short-cut fibers of a core-sheath type composite fiber described below were used as the short-cut thermal adhesive fiber B and the dryer temperature was changed to 150° C. The physical properties of the obtained nonwoven fabric (filter material) are shown in Tables 1 and 2.

The fiber-forming component was polyethylene terephthalate copolymerized with 20 mol% isophthalic acid (intrinsic viscosity 0.64 dL/g, melting point 202°C), and the thermal adhesive component was polyethylene terephthalate copolymerized with 20 mol% isophthalic acid and 65 mol% 1,4 butanediol (intrinsic viscosity 0.62 dL/g, melting point (softening point) 155°C). Short-cut thermal adhesive fiber B was produced using a core-sheath type composite fiber with the fiber-forming component as the core and the thermal adhesive component as the sheath.

The amount of the copolymerized isophthalic acid component contained in short-cut thermal adhesive fiber B was 20 mol%, calculated as the sum of 10 mol% (50% of 20 mol%) contained in the fiber-forming component and 10 mol% (50% of 20 mol%) contained in the thermal adhesive component.

To obtain short-cut thermally adhesive fiber B, the fiber-forming component and the thermally adhesive component were each melted in separate vent-type twin-screw extruders, and then the fiber-forming component was used as the core component and the thermally adhesive component was used as the sheath component. The fiber-forming component was used as the core component, ... and the thermally adhesive component were then combined in a mass ratio of core:sheath = 50:50 using a known core-sheath composite spinneret with 1,336 capillaries with a hole diameter of 0.3 mm, and the melt was extruded into a filament. The nozzle temperature was 290°C, and the extrusion rate was 430 g/min.

The extruded yarn was then cooled and solidified with 25°C cooling air at a position 42 mm below the nozzle, and then wound up at 500 m/min while applying a polyether-polyester copolymer aqueous emulsion at a solids deposition rate of 0.5% by mass below the extruded yarn, to obtain an undrawn yarn.

This undrawn yarn was stretched 28 times in warm water at 82°C, and then stretched 2.9 times in warm water at 70°C. After applying a polyether-polyester copolymer aqueous solution to the fiber surface, it was cut to a fiber length of 3 mm to obtain short-cut heat-adhesive fiber B with a fineness of 0.079 dtex (fiber diameter 2.8 μm, fiber length 3 mm, aspect ratio 1067).

[Example 4]
A wet-laid nonwoven fabric was obtained in the same manner as in Example 3, except that the basis weight in the papermaking process in Example 3 was changed to 300 g/ ^m2 . The physical properties of the obtained nonwoven fabric (filter material) are shown in Tables 1 and 2.

[Examples 5 and 6]
Polyethylene terephthalate copolymerized with 20 mol% isophthalic acid (intrinsic viscosity 0.64 dL/g, melting point 202°C) was melted in a vented twin-screw extruder and extruded in the form of a filament from a known single-component spinneret having 1192 capillaries with a hole diameter of 0.18 mm. The spinneret temperature was 290°C and the extrusion rate was 200 g/min.

The extruded yarn was then cooled and solidified at a position 26 mm below the nozzle by cooling air at 25°C, and then wound up at 500 m/min while applying 0.5% by mass of a polyether-polyester copolymer aqueous emulsion in terms of solid content to the lower part of the yarn to obtain an undrawn yarn. This undrawn yarn was stretched 68 times in warm water at 82°C, and then stretched twice in warm water at 70°C. After applying an aqueous polyether-polyester copolymer solution to the fiber surface, the fiber was cut to a fiber length of 3 mm to obtain a short-cut heat-adhesive fiber B consisting only of a heat-adhesive component with a fineness of 0.025 dtex (fiber diameter 1.6 μm, fiber length 3 mm, aspect ratio 1897). The copolymerized isophthalic acid component contained in the short-cut heat-adhesive fiber B was 20 mol%.

In Examples 1 and 2, the short-cut heat-adhesive fiber B was replaced with the one described above, and the paper was made and dried in the same manner as in Examples 1 and 2. The paper was then subjected to a heat-pressing treatment at 190°C, 29 kN/m, and a speed of 2 m/min using a hydraulic clearance embossing machine (manufactured by Yuri Roll Co., Ltd.) to obtain a wet-laid nonwoven fabric. The physical properties of the obtained nonwoven fabric (filter material) are shown in Tables 1 and 2 and Tables 3 and 4.

[Comparative Example 1]
A wetlaid nonwoven fabric was obtained in the same manner as in Example 1, except that a core-sheath type composite fiber described below was used as the short-cut thermal adhesive fiber B. The physical properties of the obtained nonwoven fabric (filter material) are shown in Tables 3 and 4.

The fiber-forming component is polyethylene terephthalate (intrinsic viscosity 0.64 dL/g, melting point 256°C), and the thermal adhesive component is 40 mol% isophthalic acid-4 mol% diethylene glycol copolymerized polyethylene terephthalate (intrinsic viscosity 0.55 dL/g, melting point (softening point) 110°C). The fiber-forming component is a core and the thermal adhesive component is a sheath. Using this core-sheath composite fiber, short-cut thermal adhesive fiber B was prepared.

The method for producing the core-sheath composite fiber was to melt the fiber-forming component and the thermal adhesive component in separate vent-type twin-screw extruders, combine the fiber-forming component as the core component and the thermal adhesive component as the sheath component in a mass ratio of core:sheath = 50:50, and extrude the melted fibers into a filament using a known core-sheath composite spinneret with 1,336 capillaries with a hole diameter of 0.3 mm. The nozzle temperature was 290°C and the extrusion rate was 600 g/min.

The extruded yarn was then cooled and solidified at a position 56 mm below the nozzle by cooling air at 25°C, and below that, a polyether-polyester copolymer aqueous emulsion was applied in an amount of 0.5 mass% in terms of solid content adhesion while being wound at 1350 m/min to obtain an undrawn yarn. This undrawn yarn was stretched 2.6 times in warm water at 58°C, and then stretched 1.15 times in warm water at 56°C. After applying the polyether-polyester copolymer aqueous solution to the fiber surface, it was cut to a fiber length of 5 mm to obtain short-cut heat-adhesive fiber B with a fineness of 1.1 dtex (fiber diameter 10.5 μm, fiber length 5 mm, aspect ratio 477).

The copolymerized isophthalic acid component contained in this short-cut thermally adhesive fiber B was 20 mol%, calculated as the sum of 0 mol% (50 parts by mass of 0 mol%) contained in the fiber-forming component and 20 mol% (50 parts by mass of 40 mol%) contained in the thermally adhesive component.

[Comparative Example 2]
A wet-laid nonwoven fabric was obtained in the same manner as in Comparative Example 1, except that the basis weight was 300 g/ ^m2 in the papermaking process of Comparative Example 1. The physical properties of the obtained nonwoven fabric (filter material) are shown in Tables 3 and 4.

[Comparative Examples 3 and 4]
A wet-laid nonwoven fabric was obtained in the same manner as in Examples 5 and 6, using fibers consisting of a single component of the thermal adhesive component described below as short-cut thermal adhesive fiber B. The physical properties of the obtained nonwoven fabric (filter material) are shown in Tables 3 and 4.

To obtain fibers consisting of a single thermal adhesive component, polyethylene terephthalate copolymerized with 20 mol% isophthalic acid (intrinsic viscosity 0.64 dL/g, melting point 202°C) was melted in a vented twin-screw extruder and extruded in the form of a melt thread from a known single-component spinneret with 1,192 capillaries with a hole diameter of 0.18 mm. The nozzle temperature was 285°C and the extrusion rate was 180 g/min.

The extruded yarn was then cooled and solidified at 25°C with cooling air at a position 25 mm below the nozzle, and then wound up at 1350 m/min while applying 0.5% by mass of polyether-polyester copolymer aqueous emulsion in terms of solid content below the yarn to obtain an undrawn yarn. This undrawn yarn was not drawn, but an aqueous polyether-polyester copolymer solution was applied to the fiber surface, and then cut to a fiber length of 5 mm to obtain a short-cut heat-adhesive fiber B consisting of a single heat-adhesive component with a fineness of 1.1 dtex (fiber diameter 10.5 μm, fiber length 5 mm, aspect ratio 477). The amount of copolymerized isophthalic acid component contained in this short-cut heat-adhesive fiber B was 20 mol%.

The numbers in parentheses in the melting point column indicate the softening point.
Tables 1 to 4 are compiled and presented below as Table 5.

The nonwoven fabric of the present invention can be suitably used as a filter medium. This filter has high collection efficiency, low pressure loss, and a long filter life, so it can be suitably used as an air filter for intake of internal combustion engines. Of course, it can also be used as an air filter, microfilter, or liquid filter for other purposes such as room air conditioners, coolers, heaters (electric, kerosene, etc.), automobile air conditioners, air purifiers, clean rooms, and room humidifiers.

Reference Signs List 1 Spinneret 2 Island component polymer reservoir 3 Island component polymer introduction passage 4 Sea component polymer introduction passage 5 Sea component polymer reservoir 6 Core-sheath type composite flow passage 7 Junction passage 8 Discharge port

Claims (6)

A nonwoven fabric comprising short-cut ultrafine fibers having a fiber diameter of 0.1-1 μm and an aspect ratio of 100-3000, short-cut heat-adhesive fibers having a fiber diameter of 0.5-4 μm and an aspect ratio of 100-3000, and polyethylene terephthalate short fibers having a fineness of 0.1-0.6 dtex and a fiber length of 1-10 mm, the short-cut heat-adhesive fibers being composite fibers made of a fiber-forming component and a heat-adhesive component, the fiber-forming component occupying 20-80% by mass of the composite fiber, the heat-adhesive component occupying 80-20% by mass, the fiber-forming component being a polyester having a melting point of 180°C or higher, the heat-adhesive component being a polyester having a melting point 20°C or higher lower than the melting point of the fiber-forming component, and having a compressibility of 30% or less when pressurized with 30 kPa. The nonwoven fabric according to claim 1, wherein the short-cut heat-adhesive fibers are made of polyester, both the fiber-forming component and the heat-adhesive component are copolymerized with an isophthalic acid component, and the weighted average copolymerization amount of the isophthalic acid component in both components is in the range of 10 to 40 mol %. The nonwoven fabric according to claim 1, wherein the composite fiber is a core-sheath type composite fiber having a fiber-forming component as the core and a thermal adhesive component as the sheath. The nonwoven fabric according to claim 1 or 2, in which the thermal adhesive component consists solely of polyester having a melting point of 250°C or less. The nonwoven fabric according to claim 1, in which the content of short-cut ultrafine fibers having a fiber diameter of 0.1 to 1 μm and an aspect ratio of 100 to 3000 is 0.5 to 50 mass %, the content of short-cut heat-adhesive fibers having a fiber diameter of 0.5 to 4 μm and an aspect ratio of 100 to 3000 is 10 to 99.5 mass %, the basis weight is 1 to 500 g/m2, the thickness is 0.01 to 3.0 mm, the average pore diameter is 0.1 to 10.0 μm, and the value of maximum pore diameter/average pore diameter is 1.0 to 2.5. A filter comprising the nonwoven fabric according to any one of claims 1 to 5.