US20190292699A1

US20190292699A1 - Fiber material and method for manufacturing the same

Info

Publication number: US20190292699A1
Application number: US16/439,592
Authority: US
Inventors: Tetsuo Hino; Kazuhiro Yamauchi
Original assignee: Canon Inc
Current assignee: Canon Inc
Priority date: 2014-01-27
Filing date: 2019-06-12
Publication date: 2019-09-26
Also published as: JP6494162B2; JP2015140492A; US20150211175A1

Abstract

A fiber material contains an assembly of a plurality of nanofiber coated polymer fibers in which a polymer fiber serving as a core fiber is coated with polymer nanofibers, in which the polymer fibers and the polymer nanofibers are fusion-bonded and two or more of the polymer fibers are fusion-bonded to each other in at least one portion.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No. 14/604,489, filed Jan. 23, 2015, which claims the benefit of Japanese Patent Application No. 2014-012756, filed Jan. 27, 2014, each of which is hereby incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a fiber material and a method for manufacturing the same and more specifically relates to a fiber material coated with nanofiber and a method for manufacturing the same.

Description of the Related Art

In recent years, a fiber material has drawn attention as a material having a large specific surface area. In particular, an increase in specific surface area based on surface treatment of fibers and a development of a material employing the same have been energetically examined in and outside Japan.
Japanese Patent Laid-Open No. 2009-219952 discloses a fiber material for filters having a large specific surface area in which nanofibers to which a functional group is given are ejected to a core fiber while applying a high voltage to coat the core fiber. Herein, an increase in specific surface area is achieved by performing surface treatment of fibers by coating with nanofibers. More specifically, the fiber material coated with nanofibers (nanofiber coated fiber material) which is manufactured by spouting the nanofibers to the core fiber while winding a thin fiber serving as the core fiber has a large specific surface area. Therefore, it is described that a bobbin type filter having excellent filtration performance is obtained by forming fiber bundles by twisting and bundling a plurality of nanofiber coated fiber materials, and then winding the fiber bundles around the periphery of a cylindrical body having a cylinder wall with transmission properties.
However, the nanofiber coated fiber material disclosed in Japanese Patent Laid-Open No. 2009-219952 has a constitution that the core fiber and the nanofibers coating the same or the core fibers are physically twisted. Therefore, the nanofiber coated fiber material has had problems in the following application. First, the peeling resistance between the nanofibers and the core fiber is low, and thus the nanofibers are easily peeled and detached due to an external factor, such as rubbing, so that the specific surface area of the fiber material decreases. Second, the mechanical strength of the fiber material is low, and thus the fibers are easily loosened, which is disadvantageous for long-term use.

SUMMARY OF THE INVENTION

The present invention provides a fiber material containing an assembly of a plurality of nanofiber coated polymer fibers in which a polymer fiber serving as a core fiber is coated with polymer nanofibers, in which the polymer fiber and the polymer nanofibers are fusion-bonded and two or more of the polymer fibers are fusion-bonded to each other in at least one portion.
The present invention also provides a method for manufacturing a fiber material including a spinning process including coating a polymer fiber as a core fiber containing a polymer material with polymer nanofibers containing a polymer material to obtain a nanofiber coated polymer fiber, and a fusion-bonding process including fusion-bonding the polymer fiber and the polymer nanofibers and also fusion-bonding two or more of polymer fibers in at least one portion.
Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are schematic views illustrating one embodiment of a fiber material of the present invention.

FIG. 2 is a schematic view illustrating one embodiment of a method for manufacturing a fiber material of the present invention.

FIG. 3 is an optical microscope photograph of a fiber material of Example 5 of the present invention.

DESCRIPTION OF THE EMBODIMENTS

The present invention has been made in view of such a background art and provides a fiber material coated with nanofibers which is excellent in peeling resistance between nanofibers and a core fiber and has high mechanical strength and a method for manufacturing the same.
Hereinafter, embodiments of the present invention are described.

Fiber Material According to the Present Invention

First, a fiber material according to the present invention is described.
The present invention provides a fiber material containing an assembly of a plurality of nanofiber coated polymer fibers in which a polymer fiber serving as a core fiber is coated with polymer nanofibers, in which the polymer fibers and the polymer nanofibers are fusion-bonded and two or more of the polymer fibers are fusion-bonded to each other in at least one portion.
For example, the fiber material according to the present invention containing an assembly of a plurality of nanofiber coated polymer fibers formed by coating a polymer nanofiber as a core fiber with polymer nanofibers, in which the polymer fiber and the polymer nanofibers coating the polymer fibers are fusion-bonded and two or more of the polymer fibers forming the fiber material are fusion-bonded to each other in at least one portion.
With respect to the fiber diameter of the core fiber and the coating fibers coating the core fiber in the present invention, the average diameter of the fibers is 1 μm or more and 50 μm or less and 1 nm or more and less than 1 μm, respectively. For the core fiber, known fiber materials, e.g., carbon fiber materials and fiber materials, can be used singly or in combination of two or more kinds thereof as appropriate.
In this embodiment, among the fibers, fibers containing polymer molecules and having an average diameter of 1 μm or more and 50 μm or less are referred to as polymer fibers, fibers having an average diameter of 1 nm or more and less than 1 μm are referred to as nanofibers, and, in particular, fibers containing polymer molecules and having an average diameter 1 nm or more and less than 1 μm are referred to as polymer nanofibers.
The fibers according to the embodiment of the present invention have a length longer than the thickness of the fibers. The cross-sectional shape of the fibers is not particularly limited and may be a circle, an oval, a quadrangle, a polygon, a semicircle, and the like or may not be an exact shape and may have different shapes in an arbitrary cross section.
The thickness (diameter) of the fibers refers to the diameter of the circle of the cross section in a fiber in which the cross-section of the fiber is a cylindrical shape and, in other cases, refers to the length of the longest straight line passing through the center of gravity in the fiber cross section. The length of the polymer nanofiber is 10 times or more the thickness thereof.
A structure containing fibers has a feature that the specific surface area increases with a reduction in diameter. Therefore, the structure is effective because when the fiber diameter is smaller, a fiber material having a large specific surface area is obtained. In addition thereto, nanofibers having a nanosized diameter demonstrate an effect referred to as supramolecular arrangement effect in which, in a process of manufacturing the nanofibers, additives added to the nanofibers are uniformly arranged in the longitudinal direction in an ultrafine space and also, in the case where the nanofibers are polymer molecules (polymer nanofibers), the molecular chains of the polymer molecules are uniformly arranged. As a result, fibers having high mechanical strength are obtained.
Next, the fiber material coated with the polymer nanofibers of the present invention is described with reference to FIGS. 1A and 1B.
FIGS. 1A and 1B are schematic views illustrating one embodiment of the fiber material of the present invention. FIG. 1A illustrates a schematic cross sectional view of one nanofiber coated polymer fiber and FIG. 1B illustrates a perspective view of a fiber material. The figures include a nanofiber coated polymer fiber 1, a polymer fiber 2, a polymer nanofiber 3, and a fiber material 4.
The fiber material 4 coated with the nanofibers of this embodiment has inevitably unevenness on the surface and contains the nanofiber coated polymer fibers 1, and therefore has a large specific surface area. One of the nanofiber coated polymer fibers 1 in the fiber material of the present invention has a constitution in which the polymer nanofibers 3 at least containing a polymer material are fusion-bonded to the surface of the core fiber 2 at least containing a polymer material. As a result, the peeling resistance between the polymer nanofibers 3 and the polymer fiber 2 as the core fiber is high, so that the polymer nanofibers 3 are not easily peeled and detached due to an external factor, such as rubbing, to reduce the specific surface area of the fiber material.
The specific surface area and the surface unevenness in the nanofiber coated polymer fibers depend on the coating ratio of the polymer nanofibers, the number of the polymer nanofibers coating the polymer fibers, and the like, which may be selected as appropriate according to a desired property.
In the fiber material of the present invention, the number, the interval of adjacent fibers, and the number of laminations of the nanofiber coated polymer fibers 1 in an arbitrary cross section can be suitably selected according to a desired property of the fiber material. For example, FIG. 1B illustrates a constitution in which a plurality of nanofiber coated polymer fibers 1 are disposed in a random shape and the nanofiber coated polymer fibers 1 are fusion-bonded to each other in at least one portion.
More specifically, a plurality of nanofiber coated polymer fibers adjacent to each other which form the fiber material 4 form a firm and flexible network by fusion-bonding to each other in at least one portion. As a result, the fiber material obtained by the present invention has high mechanical strength, is free from ease loosening of the core fibers, and is advantageous for long-term use.
From the description above, the present invention can provide the fiber material in which the polymer nanofibers coat the polymer fiber in which the peeling resistance between the polymer fiber as the core fiber and the polymer nanofibers is high, the mechanical strength of the fiber material is high, and the specific surface area is large.
The fiber material of the present invention can be a fiber material having a large specific surface area which can be used over a long period of time even when an external factor, rubbing, is applied, and therefore can be suitably utilized, for example, as a friction charging material in a static electricity generator and a particle electric field separator.

Polymer Fiber as Core Fiber

The polymer fiber as the core fiber in the present invention is not particularly limited and may be a fiber at least containing a polymer material, and an organic polymer is suitable. As the organic polymer, known organic polymer materials can be used singly or in combination as appropriate. Moreover, the organic polymer may be a polymer material containing fine particles or a known filler and the like and can be constituted by combining the same as appropriate.
The polymer fiber in the present invention contains at least one or more kinds of polymers and has a length longer than the thickness of the polymer fiber.
The thickness of the polymer fiber is suitably larger than that the polymer nanofiber coating the polymer fiber and the average diameter is suitably 1 μm or more and 50 μm or less. The average diameter is more suitably 10 μm or more and 50 μm or less.
In order to coat the polymer fiber with the polymer nanofibers, when the core fiber is thinner than the polymer nanofibers, the handling is difficult from the viewpoint of manufacturing and also it is sometimes difficult to excellently coat the periphery of the core fiber with the polymer nanofibers. Therefore, the thickness of the polymer fiber is suitably larger than that of the polymer nanofiber for coating the polymer fiber and more suitably 10% or more larger than that of the polymer nanofibers and the average diameter is suitably 1 μm or more.
In addition thereto, in order to obtain a fiber material having a large specific surface area, the average diameter of the polymer fiber is suitably 50 μm or less because a large specific surface area tends to be easily obtained by coating the core fiber with the polymer nanofibers with a small diameter.
The cross-sectional shape of the polymer fiber is not particularly limited and may be a circle, an oval, a quadrangle, a polygon, a semicircle, and the like or may not be an exact shape and may have different shapes in an arbitrary cross section.
The thickness of the polymer fiber refers to the diameter of the circle of the cross section in one in which the cross section of the polymer nanofiber has a circular shape but, in other cases, the thickness refers to the length of the longest straight line passing through the center of gravity in the fiber cross section. The length of the polymer nanofibers is 10 or more times the thickness thereof.
The polymer material of the polymer fibers in the present invention is not particularly limited insofar as the material can at least partially melt and forms a fiber structure, and organic materials, such as resin materials, inorganic materials, such as silica, titania, and clay mineral, or materials obtained by hybridizing the organic materials and the inorganic materials may be used.
Examples of the polymer material include, for example, polyolefin polymers, such as fluorine containing polymers, e.g., tetrafluoroethylene and polyvinylidene fluoride, for example, polyvinylidene fluoride (PVDF), a copolymer (PVDF-HFP) of PVDF and hexafluoro propylene, polyethylene, and polypropylene; polystyrene (PS); polyarylenes (aromatic polymers), such as polyparaphenylene oxide, poly(2,6-dimethyl phenylene oxide), and polyparaphenylene sulfide; those in which a sulfonic acid group (—SO₃H), a carboxyl group (—COOH), a phosphoric group, a sulfonium group, an ammonium group, or a pyridinium is introduced into polyolefin polymers, polystyrenes, polyimides, and polyarylenes (aromatic polymers); perfluoro sulfonic acid polymers, perfluoro carboxylic acid polymers, and perfluoro phosphoric acid polymers in which a sulfonic acid group, a carboxyl group, a phosphoric group, a sulfonium group, an ammonium group, or a pyridinium group is introduced into the skeleton of polymers of polytetrafluoroethylene and fluorine containing polymers, polybutadiene compounds; polyurethane compounds, such as elastomer or gel; silicone compounds; polyvinyl chlorides; polyethylene terephthalate; nylon; and polyesters (PES), such as polyarylate, polycapro lactone (PCL) and polylactic acid which are biodegradable polymers, ethers, such as polyethylene oxide (PEO) and polybutylene oxide, and polyethylene terephthalate (PET). These substances can be used singly or in combination of two or more kinds thereof, may be functionalized, or may be copolymerized with other polymers. In the case of polymer materials, such as polyimide, polyamide, polyamide imide (PAI), and polybenzimidazole (PBI) which are hard to be melted, the polymer materials can be used in combination with thermoplastic resin, for example.
Examples of the inorganic materials include oxides of Si, Mg, Al, Ti, Zr, V, Cr, Mn, Fe, Co, Ni, Cu, Sn, and Zn and, more specifically, the following metal oxides are mentioned. Silica (SiO₂), titanium oxide, aluminum oxide, alumina sol, zirconium dioxide, iron oxide, chromium oxide, and the like can be mentioned. Clay mineral, such as montmorillonite (MN), can also be used.
When the inorganic materials are contained in the polymer fibers, the mechanical strength tends to remarkably improve by fusion-bonding the fibers. Therefore, the blending of the inorganic materials is suitable from the viewpoint of an improvement of durability.
In the present invention, when the polymer fibers at least contain the polymer material constituting the polymer nanofibers, the compatibility thereof is high, and thus the fibers tend to be firmly fusion-bonded.
The description “the polymer fibers at least contain the polymer material constituting the polymer nanofibers” in the present invention means that polymer material formed with the same skeleton structure as the skeleton constituting the polymer nanofibers may be at least contained in the polymer fibers and it is not necessary to be the same polymer material.
Due to the material configuration described above, there is a tendency for the mechanical strength of the fiber material in the present invention to remarkably improve. Therefore, the material configuration is suitable from the viewpoint of an improvement of durability.
When the melting temperature (Melting point: Tm₁) forming the polymer fiber is less than Tm₂of the polymer material forming the polymer nanofibers and the difference is large, the polymer fibers and the polymer nanofibers coating the same can be fusion-bonded by preferentially melting the polymer fiber side. More specifically, the shape (unevenness) of the polymer nanofibers is maintained and an effect of increasing the specific surface area in the fiber material based on the coating with the polymer nanofibers is easily demonstrated. Therefore, the fusion-bonding manner is suitable.
Herein, it is suitable that the temperature difference (Tm₂−Tm₁) in the melting points of the polymer fibers and the polymer nanofibers is 5° C. or higher and suitably 30° C. or higher from the viewpoint of thermal control. More specifically, when the temperature difference is less than 5° C., the shape of the polymer nanofibers tends to be difficult to maintain.
It is more suitable that the melting point (Tm) of the polymer material forming the polymer fibers is equal to or less than the glass transition point (Tg) of the polymer material forming the polymer nanofibers because the shape of the polymer nanofibers is very easily maintained in the fusion-bonding process.
When the polymer fiber or the polymer nanofiber each is formed from a plurality of polymer materials in the present invention, the Tm and the Tg of each composite material is a lower Tm value of Tm values of the fibers and a lower Tg value of Tg values of the fibers.
It is also suitable for the polymer fibers to at least contain materials having high sharp melt properties from the viewpoint of ease of handling in the fusion-bonding process. As the materials having high sharp melt properties, known substances can be used singly or in combination of two or more kinds thereof as appropriate and, for example, a hot melt agent and a low melt agent can also be suitably used. Specifically, as the hot melt agent, PES310S30, PES360S30, and PES375S40 (all manufactured by TOAGOSEI CO., LTD.), 3738, 3747, 3762, and 3764 (all manufactured by 3M), and the like can be used singly or in combination. As the low melt agent, 3762LM, 3776LM, 3792LM, and 3798LM (all manufactured by 3M) and the like can be used singly or in combination.

Polymer Nanofiber as Coating Fiber

The materials of the polymer nanofibers as the coating fibers in the present invention are not particularly limited and may be fibers at least containing polymer materials and organic polymers are suitable. As the organic polymers, known polymer materials can be used as appropriate. Polymer materials containing fine particles or a known filler and the like may be acceptable and the materials can be constituted combining the same as appropriate.
The polymer nanofibers according to an embodiment of the present invention have at least one or more kinds of polymers and have a length longer than the thickness of the polymer nanofibers.
The polymer nanofibers are suitably thinner than the polymer fiber as the core fiber. With respect to the thickness of the polymer nanofibers, the average diameter is suitably 1 nm or more and less than 1 μm from the viewpoint of coating the core fiber to increase the specific surface area. The average diameter is more suitably 30 nm or more and less than 1 μm.
In coating the peripheral surface of the core fiber with the polymer nanofibers, when the polymer nanofibers are thicker than the core fiber, handling of the polymer nanofibers is difficult from the viewpoint of manufacturing, which makes it difficult to coat the peripheral surface of the core fiber with the polymer nanofibers again in some cases. Therefore, the polymer nanofibers are suitably thinner than the polymer fiber as the core fiber and suitably have an average diameter of less than 1 μm and, more specifically, those which are 10% or more thinner than the polymer fibers are suitable.
In general, the polymer nanofibers having an average diameter of less than 1 nm need to be manufactured by special techniques, such as a self-assembly method and a phase separation method, and the cost for mass production thereof tends to be high. Therefore, the thickness of the polymer nanofibers is suitably 1 nm or more.
The cross-sectional shape of the polymer nanofibers is not particularly limited and may be a circle, an oval, a quadrangle, a polygon, a semicircle, and the like or may not be an exact shape and may have different shapes in an arbitrary cross section.
The thickness of the polymer nanofibers refers to the diameter of the circle of the cross section in one in which the cross section of the polymer nanofibers has a circular shape but, in other cases, the thickness refers to the length of the longest straight line passing through the center of gravity in the polymer nanofiber cross section. The length of the polymer nanofibers is 10 or more times the thickness thereof.
The polymer material to be used in the polymer nanofibers according to the present invention is not particularly limited but is limited only in terms of the relationship with the polymer material to be used as the polymer fibers described above and the polymer material of the polymer fibers can be used as appropriate.
It is suitable for the polymer nanofibers coating the polymer fibers to at least contain the polymer material constituting the polymer fibers.
From the viewpoint of maintaining the shape of the polymer nanofibers in the fusion-bonding process, it is also suitable to use polymer materials, such as polyimide, polyamide, polyamide imide (PAI), and polybenzimidazole (PBI) which are hard to be melt, singly without combining with thermoplastic resin and the like, for example. More specifically, polyimide, polyamide, polyamide imide (PAI), polybenzimidazole (PBI), and the like are referred to as an engineering plastic and have a Tg higher than the Tm of general thermoplastic polymer materials. Therefore, the use of these substances is suitable because the shape of the polymer nanofibers tends to easily maintain in the fusion-bonding process.

Coating Amount of Polymer Nanofibers Coating Polymer Fiber

The amount of the polymer nanofibers coating the polymer fiber as the core fiber may be adjusted as appropriate according to desired performance and, for example, is 1 part by weight or more and 95 parts by weight or less and suitably 5 parts by weight or more and 90 parts by weight or less based on 100 parts by weight of the polymer fibers. The amount of the polymer nanofibers of less than 1 part by weight is not suitable because an increase in specific surface area is insufficient in some cases. When the amount of the polymer nanofibers exceeds 95 parts by weight, the polymer fibers cannot be fusion-bonded to each other in at least one portion in some cases, and therefore the amount is not suitable.

Method for Manufacturing Fiber Material According to the Present Invention

Next, a method for manufacturing the fiber material according to the present invention is described.
The method for manufacturing the fiber material includes a spinning process including coating a polymer fiber as a core fiber containing a polymer material with polymer nanofibers containing a polymer material to obtain a nanofiber coated polymer fiber, and a fusion-bonding process including fusion-bonding the polymer fiber and the polymer nanofibers and also fusion-bonding two or more of the polymer fibers in at least one portion.
Specifically, the method for manufacturing the fiber material according to the present invention includes a spinning process including coating a polymer fiber as a core fiber containing a polymer material with polymer nanofibers containing a polymer material to obtain a nanofiber coated polymer fiber and a fusion-bonding process including fusion-bonding the polymer fiber and the polymer nanofibers coating the polymer fibers and also fusion-bonding two or more of polymer fibers forming the fiber material in at least one portion.
The method for manufacturing the fiber material according to the present invention is not particularly limited and, for example, a method including performing bicomponent spinning of the polymer fibers and the polymer nanofibers by an electrospinning method (an electric field spinning method and an electrostatic spinning method), and then heating the resultant fibers in an oven for fusion-bonding the fibers to each other and the polymer nanofibers and the polymer fibers or, in place of the bicomponent spinning using the electrospinning method singly, the electrospinning method, a melt blow method, and the like may be used in combination.
The electrospinning method is a method for manufacturing polymer fibers in which a high voltage is applied between a polymer solution in a syringe and a collector electrode to charge the solution forced out of the syringe, and then the solution is dispersed to be formed into thin lines to form polymer nanofibers, and the polymer nanofibers adhere to the collector.
Among the manufacturing methods described above, it is suitable to perform the bicomponent spinning by the electrospinning method because spinning of various polymers into a fiber shape can be performed, control of the fiber shape is relatively easy, fibers having an average diameter ranging from several tens μm to a nanosize can be easily obtained, and also the manufacturing process is simple.
The spinning process using the bicomponent spinning of the polymer fibers and the polymer nanofibers by the electrospinning method which is one embodiment of the present invention is described with reference to FIG. 2.
FIG. 2 is a schematic view illustrating one embodiment of the method for manufacturing the fiber material of the present invention. In particular, FIG. 2 is a schematic view illustrating the bicomponent spinning using the electrospinning method which is an example of a technique for manufacturing the nanofiber coated polymer fiber of the present invention.
As illustrated in FIG. 2, the bicomponent spinning is performed using a head 17 in which a plurality of polymer solution storage tanks 12, 13, and 14 are disposed through a connection portion 11 connected to a high-pressure power supply 16 and a grounded collector 15. The reference numeral 10 denotes a device for manufacturing the fiber material.
The polymer solution is forced out of the tanks 12, 13, and 14 to spinning orifices 22, 23, and 24 at a fixed speed. A voltage of 1 to 50 kV is applied to each of the spinning orifices 22, 23, and 24. When electric attraction exceeds the surface tension of the polymer solution, polymer solution jets 18, 19, and 20 are ejected to the collector 15. In this process, the solvent in the jets is gradually volatilized, and then, when reaching the collector, corresponding fibers are obtained. Herein, the polymer solutions whose conditions are set to the conditions under which the polymer solutions are formed into polymer nanofibers are introduced into the tanks 12 and 13, and, on the other hand, a polymer solution whose conditions are set to the conditions under which the polymer solution is formed into polymer fibers is introduced into the tank 14, and then bicomponent spinning is performed.
Herein, a melted polymer heated to be equal to or higher than the melting point may be utilized instead of the polymer solution.
The polymer nanofibers and the polymer fibers can be confirmed by direct observation using a scanning electron microscope (SEM) or laser microscope measurement.
The average fiber diameter (average diameter) of the polymer nanofibers or the polymer fibers can be determined by measuring the concerned composite fiber film under a scanning electron microscope (SEM), capturing an image thereof into image analysis software “Image J”, and then measuring the width at arbitrary 50 points of the polymer nanofibers or the polymer fibers.

Fusion-Bonding Process of Polymer Nanofibers and Polymer Fibers, and Polymer Fibers

The description “fusion-bonding the polymer fibers and the polymer nanofibers coating the polymer fibers” or “fusion-bonding the plurality of polymer fibers forming the fiber material to each other in at least one portion” in the present invention refers to a state where at least the polymer fibers are softened to adhere to the polymer nanofibers coating the polymer fibers or adjacent polymer fibers, so that the adhesion boundary portion has a sheet shape or the adhesion boundary is lost.
The fusion-bonding process is not particularly limited and may be thermal fusion-bonding, ultrasonic fusion-bonding, and friction fusion-bonding and also fusion-bonding (hotpress) by thermocompression bonding and is suitably thermal fusion-bonding in terms of ease of handling. The thermal fusion-bonding method is not particularly limited and, for example, a hot pressing method, a method for performing fusion-bonding by heating with an industrial drier, oven, or the like, a method for performing heating with a heater once, and then further heating in an oven for fusion-bonding, and the like can be suitably used. Among the above, a technique of performing fusion-bonding using an oven can be particularly suitably used because the temperature of the entire material can be easily made uniform without unevenness.
The temperature of the fusion-bonding is not particularly limited insofar as the temperature is equal to or less than the decomposition temperature of the polymer material and may be selected as appropriate according to the polymer material to be used, a desired fiber material, and the like. The temperature of heating is suitably 30 to 250° C. and more suitably near the melting point of at least the polymer fibers or both the polymer fibers and the polymer nanofibers. As described above, when the temperature of the fusion-bonding is equal to or higher than the melting point (Tm) of the polymer material forming the polymer fibers and is equal to or less than the glass transition point (Tg) of the polymer material forming the polymer nanofibers, the shape of the polymer nanofibers is easily maintained and thus the temperature of the fusion-bonding is very suitable. The heating time is suitably 0.1 minute to 60 minutes.
A method for confirming “the fusion-bonding of the polymer fibers and the polymer nanofibers coating the polymer fibers” and “the fusion-bonding of the polymer fibers (the polymer fibers are fusion-bonded to each other in at least one portion)” can be performed in direct observation using a laser microscope or an electron microscope (SEM) before and after the fusion-bonding process.
More specifically, the polymer fibers are fusion-bonded to each other in at least one portion, i.e., the fusion-bonding of the polymer fibers in at least one portion, in the present invention refers to a state where the polymer fibers are softened to adhere to adjacent polymer fibers, so that the adhesion boundary portion has a sheet shape or the adhesion boundary is lost. Therefore, when the state is confirmed in which the polymer fibers or the polymer nanofibers are softened, so that the adjacent polymer fibers or the polymer fibers and the polymer nanofibers coating the same adhere to each other, so that the adhesion boundary portion has a sheet shape or the adhesion boundary is lost, it can be confirmed that the fibers are fusion-bonded.
FIG. 3 shows an optical microscope photograph of a nanofiber coated polymer fiber material of Example 5 as an example showing the fusion-bonding of the polymer nanofibers and the polymer fibers and the fusion-bonding of the polymer fibers according to the present invention. In FIG. 3, it can be confirmed that polymer nanofibers having an average diameter of about 700 nm formed from polyamide imide (PAI) are fusion-bonded to the surface of the polymer fibers having an average diameter of about 35 μm formed from a polyester material (PES) to coat the polymer fibers and adjacent polymer fibers have a structure that the polymer fibers are fusion-bonded to each other in at least one portion.

Evaluation of Fusion-Bonding Degree of Polymer Fiber and Polymer Nanofibers Coating Polymer Fiber

The evaluation of the fusion-bonding degree of the polymer nanofibers coating the polymer fibers in the present invention was performed using a simple tape peeling test including bonding a pressure sensitive adhesive tape (NICHIBAN CO., LTD.: CT-18, 0.401 N/mm) to both surfaces of a manufactured fiber material, and then vertically tearing the same off using an instron tester (Shimadzu: EZ-test).
More specifically, arbitrary 10 points (observation points) on the fiber material surface were marked beforehand, the peeling degree of the polymer nanofibers coating the polymer fibers before and after the simple tape peeling test at the observation points was observed under a laser microscope, and then the peeling degree was evaluated in the following three grades of A to C:
A: At all the observation points, peeling of the polymer nanofibers coating the polymer fibers is not observed;
B: At 1 to 4 observation points, peeling of the polymer nanofibers coating the polymer fibers is observed;
C: At 5 or more observation points, peeling of the polymer nanofibers coating the polymer fibers is observed.
The fusion-bonding degree is higher in order of A>>B>C, and, as a result, the peeling resistance between the polymer nanofibers and the polymer fiber is high. Therefore, polymer nanofiber coated polymer fiber materials free from easy peeling and detachment of the polymer nanofibers due to an external factor, such as rubbing, leading to a reduction in specific surface area of the fiber material are obtained in order of A>>B>C.

Evaluation of Fusion-Bonding Degree of Polymer Fibers

An increase in mechanical strength of the fiber material due to the fusion-bonding of the polymer fibers in the nanofiber coated polymer fiber material can be easily confirmed by measuring the Young's modulus of the corresponding fiber materials before and after the fusion-bonding process.
The evaluation of the fusion-bonding degree of the polymer fibers in the present invention is performed by measuring changes in mechanical strength of the fiber materials before and after the fusion-bonding process using tensile property measurement employing an autograph (AG-Xplus) manufactured by Shimadzu.
Specifically, it is confirmed in samples before and after the fusion-bonding process that the polymer fibers are fusion-bonded to each other in at least one portion by direct observation using a laser microscope or an electron microscope (SEM). Thereafter, the Young's modulus of the fiber material in a state where the polymer nanofibers merely physically coat the polymer fibers and the Young's modulus of the samples after the fusion-bonding process are measured, and then the increase percentage of the Young's modulus is calculated.
When the increase percentage of the Young's modulus is higher, the fusion-bonding degree in which the polymer fibers are fusion-bonded to each other in at least one portion is higher. As a result, the mechanical strength of the fiber material improves, and therefore the fiber material can be used over a long period of time. Evaluation of shape changes in polymer nanofibers before and after fusion-bonding process
The shape changes in the polymer nanofibers before and after the fusion-bonding process in the present invention can be performed using a scanning electron microscope (SEM).
More specifically, the shape change rate in the polymer nanofibers is calculated by observing the fiber material before and after the fusion-bonding process observed using an SEM, capturing images thereof into image analysis software “Image J”, individually measuring the fiber widths of the polymer nanofibers at arbitrary 50 points from a direction (upper surface) perpendicular to the thickness direction of the fiber material before and after the fusion-bonding process, and then comparing the fiber widths before and after the fusion-bonding. Then, the degree of the shape change rate was evaluated in the following three stages of I to III based on the results: I: Changes in the polymer nanofibers are not observed at all the observation points;
II: Changes in the polymer nanofibers are slightly observed (within +5 to +10%);
III: Changes in polymer nanofibers are very noticeably observed (larger than +10%).

EXAMPLES

Hereinafter, Examples of the present invention are described.
Examples of the present invention are described in detail below but are merely examples and do not limit the present invention. The technique of the present invention also includes various modifications and alternations of specific examples described below.

Example 1

This example relates a fiber material in which polymer fibers are formed from polycapro lactone (PCL) and polymer nanofibers are formed from polyethylene oxide (PEO). The fiber material is manufactured as follows.
First, 1 mL of a PCL diluted solution A adjusted to 8 wt % using polycapro lactone (PCL, Weight average molecular weight of 80000, manufactured by Aldrich) and a solution obtained by mixing THF (tetrahydrofuran) and DMF (dimethyl formamide) at 6:4 (Volume ratio) is used. 2 mL of a PEO diluted solution B in which polyethylene oxide (manufactured by Aldrich) is adjusted to 6 wt % using pure water is used.
Next, these diluted solutions are simultaneously ejected for performing bicomponent spinning by an electrospinning method, whereby nanofiber coated polymer fibers in which the PCL polymer fibers as core fibers are physically coated with the PEO polymer nanofibers is obtained.
More specifically, a head (Clip spinneret, FIG. 2) capable of performing spinning of a plurality of solutions is attached to an electrospinning device (manufactured by MEC Co., Ltd.).
Next, a tank which is filled with the PCL diluted solution A is attached to the center of the head and a tank filled with the PEO diluted solution B is attached to both ends. Then, the device is moved from side to side at 50 mm/s while applying a voltage of 19 kV to spinning orifices to thereby eject each diluted solution to a collector. Then, by ejecting the solutions for 15 minutes, nanofiber coated polymer fibers in which the PCL polymer fibers are coated with the PEO polymer nanofibers can be obtained.
Then, the obtained nanofiber coated polymer fibers are held between glass plates, put into an oven, and then heat-treated at 65° C. for 0.5 minute, whereby a fiber material in which the polymer fibers and the polymer nanofibers are fusion-bonded and the polymer fibers adjacent to each other are fusion-bonded to each other in at least one portion is obtained.
The thickness of the polymer fibers in the obtained fiber material thus obtained is 18 μm in average diameter. The thickness of the polymer nanofibers is 650 nm in average diameter.

Example 2

This example is a modification of Example 1 and is manufactured in the same manner as in Example 1, except changing the materials of the polymer fibers and the polymer nanofibers and changing the following conditions.
As the material of the polymer fibers, 1 mL of a PS/PET diluted solution C in which a mixture (Volume ratio of 9:1) of polystyrene (PS, Weight average molecular weight of 280000, manufactured by Aldrich) and polyethylene terephthalate (PET, Weight average molecular weight of 10000, manufactured by Aldrich) is adjusted to a 30 wt % solution using DMF is used. As the material of the polymer nanofibers, 2 mL of a PET diluted solution D in which PET is adjusted to 13 wt % using a solution in which dichloromethane (DCM) and trifluoroacetic acid (TA) are mixed with 1:1 (volume ratio) is used.
The application of a voltage to the spinning orifices is performed at 22 kV. The fusion-bonding process is performed by heat-treatment in an oven at 260° C. for 0.5 minute, whereby a fiber material in which the polymer fibers and the polymer nanofibers are fusion-bonded and the polymer fibers adjacent to each other are fusion-bonded to each other in at least one portion is obtained.
The thickness of the polymer fibers in the obtained fiber material thus obtained is 1.5 μm in average diameter. The thickness of the polymer nanofibers is 300 nm in average diameter.

Example 3

This example is a modification of Example 1 and is manufactured in the same manner as in Example 1, except changing the materials of the polymer fibers and the polymer nanofibers and changing the following conditions.
As the material of the polymer fibers, 1 mL of a PVDF-HFP diluted solution E in which polyvinylidene fluoride-hexafluoro propylene (PVDF-HFP) (KYNAR 2750, manufactured by KYNAR) is adjusted to a 10 wt % solution using dimethyl acetamide (DMAc) is used.
As the material of the polymer nanofibers, a mixture of polyvinylidene fluoride (PVDF, KYNAR 460, manufactured by KYNAR) and montmorillonite (MN, nanoclay, manufactured by Aldrich) is used. This mixture is manufactured by the following procedure.
10 mg of montmorillonite (MN) as a filler and 1 mL of organic solvent (DMAc) are put in a container. Zirconia balls having a particle diameter of 2 mm are added to ⅓ of the capacity of the container, and then the mixture is dispersed under the conditions of 200 rpm/30 minutes using a ball mill machine (manufactured by Fritsch, Planetary pulverizer). Subsequently, a solution in which 200 mg of PVDF as a base material is dissolved in 2 mL of DMAc is added, and then further dispersed under the conditions of 500 rpm/60 minutes. Thus, 2 mL of a PVDF-MN diluted mixed solution E which is a material of polymer nanofibers is prepared.
The application of a voltage to the spinning orifices is performed at 18 kV. The fusion-bonding process is performed by heat-treatment in an oven at 155° C. for 0.5 minute, whereby a fiber material in which the polymer fibers and the polymer nanofibers are fusion-bonded and the polymer fibers adjacent to each other are fusion-bonded to each other in at least one portion is obtained.
The thickness of the polymer fibers in the obtained fiber material thus obtained is 20 μm in average diameter. The thickness of the polymer nanofibers is 220 nm in average diameter.

Example 4

This example is a modification of Example 1 and is manufactured in the same manner as in Example 1, except changing the materials of the polymer fibers and the polymer nanofibers and changing the following conditions.
The material of the polymer fibers is adjusted as follows.
In a container, 0.1 wt % of silica particles (SiO₂, nanopowder, Average particle diameter of 10 to 20 nm, manufactured by Aldrich) is added to 10 parts by weight of a thermoplastic polyester hot melt material (PES, Aronmelt PES375S40, manufactured by TOAGOSEI CO., LTD., Solid content of 40 wt %, Solvent (toluene:MEK=8:2)), and then mixed. Next, Zirconia balls having a particle diameter of 2 mm are added to ⅓ of the capacity of the container, and then the mixture is dispersed under the conditions of 500 rpm/30 minutes using a ball mill machine (manufactured by Fritsch, Planetary pulverizer) to prepare 1 mL of a PES-SiO₂diluted mixed solution E.
As the material of the polymer nanofibers, 2 mL of a PBI diluted solution F in which polybenzimidazole (PBI, manufactured by Aldrich) is adjusted to 20 wt % in a DMAc solution in which 4 wt % of lithium chloride (LiCl) is dissolved is used.
The application of a voltage to the spinning orifices is performed at 23 kV. The fusion-bonding process is performed by heat-treatment in an oven at 130° C. for 1 minute, whereby a fiber material in which the polymer fibers and the polymer nanofibers are fusion-bonded and the polymer fibers adjacent to each other are fusion-bonded to each other in at least one portion is obtained.
The thickness of the polymer fibers in the obtained fiber material thus obtained is 40 μm in average diameter. The thickness of the polymer nanofibers is 150 nm in average diameter.

Example 5

This example is a modification of Example 1 and is manufactured in the same manner as in Example 1, except changing the materials of the polymer fibers and the polymer nanofibers and changing the following conditions.
As the material of the polymer fibers, 1 mL of a thermoplastic polyester hot melt material (PES, Aronmelt PES375S40, manufactured by TOAGOSEI CO., LTD., Solid content of 40 wt %, Solvent (toluene:MEK=8:2) is prepared.
As the material of the polymer nanofibers, one in which polyamide imide (PAI, pyromax HR-13NX) is adjusted to a solid content concentration of 25 wt % using DMF is used.
The application of a voltage to the spinning orifices is performed at 19 kV. The fusion-bonding process is performed by heat-treatment in an oven at 130° C. for 1 minute, whereby a fiber material in which the polymer fibers and the polymer nanofibers are fusion-bonded and the polymer fibers adjacent to each other are fusion-bonded to each other in at least one portion is obtained.
The thickness of the polymer fibers in the obtained fiber material thus obtained is 35 μm in average diameter. The thickness of the polymer nanofibers is 700 nm in average diameter. FIG. 3 shows an optical microscope photograph of the fiber material of Example 5 of the present invention. The length between two points indicated by “1” in FIG. 3 is 33 μm.

Comparative Example 1

This comparative example is a modification of Example 1 and is manufactured in the same manner as in Example 1, except performing the fusion-bonding process in Example 1.
The thickness of the polymer fibers in the obtained fiber material thus obtained is 9 μm in average diameter. The thickness of the polymer nanofibers is 500 nm in average diameter.
Table 1 shows the melting point (Tm) and the glass transition point (Tg) of the polymer materials used in Examples and Comparative Example.

TABLE 1

	Melting	Glass transition point
Polymer	point (Tm)	(Tg)
material	(° C.)	(° C.)

PCL	60	−60
PEO	65	−73
PS	230	100
PET	260	80
PVDF-HFP	130 to 138	−42 to −40
PVDF	155 to 160	−40 to −38
PES	130	23
PBI	None	473
PAI	None	290

Evaluation of Performance of Fiber Material

It was observed by SEM that the fiber materials in Examples 1 to 5 have a structure in which the polymer fibers and the polymer nanofibers coating the polymer fibers are fusion-bonded and a plurality of polymer fibers forming the fiber materials are fusion-bonded to each other.
The peeling resistance between the nanofibers and the core fiber was examined by “Evaluation of fusion-bonding degree of polymer fiber and polymer nanofibers coating polymer fiber” performed by the tape peeling test described above.
The mechanical strength of the fiber materials was examined by “Evaluation of fusion-bonding degree of polymer nanofiber coated polymer fibers” by a tensile strength test.
The fiber materials with a large specific surface area are obtained by the coating with the polymer nanofibers. It was examined that the polymer nanofibers coating the polymer fibers are maintained before and after the fusion-bonding (maintenance of unevenness of nanofiber coated polymer fibers) by “Evaluation of shape changes in polymer nanofibers before and after fusion-bonding process”.
Table 2 shows the used materials, the peeling test results, the strength increase rate, and the degree of shape changes in the polymer nanofibers before and after the fusion-bonding process in Examples and Comparative Example.

TABLE 2

			Degree of shape
Polymer		Strength	changes in
fiber		increase rate	polymer
material		(After fusion-	nanofibers
Polymer	Peeling	bonding/Before	(After fusion-
nanofiber	test	fusion-	bonding/Before
material	results	bonding) %	fusion-bonding)

Ex. 1	PCL	B	300	II (30%)
	PEO
Ex. 2	PS + PET	A	400	II (13%)
	PET
Ex. 3	PVDF-HFP	A	600	II (10%)
	PVDF/MN
Ex. 4	PES/SiO₂	B	700	I (0%)
	PBI
Ex. 5	PES	B	650	I (0%)
	PAI
Comp.	PCL	C	—	—
Ex. 1	PEO

From the results of Examples 1 to 5 and Comparative Example 1, in Examples 1 to 5, by fusion-bonding the polymer fibers and the polymer nanofibers coating the polymer fibers, the polymer nanofibers do not peel (A) or very slightly peel (B) in the tape peeling test, so that a remarkable performance improvement of the peeling resistance between the nanofibers and the core fiber can be confirmed.
In particular, from Example 1 and Comparative Example 1, by fusion-bonding the polymer fibers and the polymer nanofibers coating the polymer fibers, a remarkable performance improvement can be confirmed in the tape peeling test (C→B) also in the same polymer fibers/polymer nanofiber material system.
In addition thereto, from Example 2 and Example 3, when the same polymer material (the structure constituting the polymer material is the same) is contained in the material of the polymer fibers and the material of the polymer nanofibers, the polymer nanofibers do not peel (A) in the tape peeling test by fusion-bonding the polymer fibers and the polymer nanofibers coating the polymer fibers, so that a remarkable performance improvement of the peeling resistance between the polymer nanofibers and the core fiber remarkably can be confirmed.
Therefore, it is found that the fiber material in the present invention has high peeling resistance between the polymer nanofibers and the core fiber. More specifically, ease peeling and detachment of the polymer nanofibers due to an external factor, such as rubbing, leading to a reduction in the specific surface area of the fiber material does not occur.

Evaluation of Fusion-Bonding Degree of Polymer Fibers

The plurality of polymer fibers forming the fiber materials of Examples 1 to 5 are structured to be fusion-bonded to each other. Therefore, it can be confirmed that the tensile strength improves several times before and after the fusion-bonding and the mechanical strength of the fiber material remarkably improves.
When a fiber material was constituted using materials, such as an inorganic filler (MN, SiO₂) or an engineering plastic (PBI, PAI), the tendency which shows a large improvement of mechanical strength before and after the fusion-bonding process was able to be confirmed (Examples 3 to 5).
Therefore, it is found that the mechanical strength of the fiber material in the present invention is high. More specifically, the core fibers are not easily loosened and the fiber material is advantageous for long-term use.
Evaluation of Shape Changes in Polymer Nanofibers Before and after Fusion-Bonding Process
It is found from the results of Examples 1 to 3 that when the melting point (Tm₁) of the polymer material forming the polymer fibers is less than the melting point (Tm₂) of the polymer material forming polymer nanofibers and the temperature difference (Tm₂−Tm₁) is 5° C. or higher (Example 1: PCL (Tm₁: 60° C.), PEO (Tm₂: 65° C.)), the shape of the polymer nanofibers before and after the fusion-bonding process is favorably maintained (II).
It can be confirmed from the results of Example 2 and Example 3 that when the temperature difference (Tm₂−Tm₁) is 30° C. or higher (Example 2: PS (Tm₁: 230° C.), PET (Tm₂: 260° C.), Example 3: PVDF-HFP (Tm₁: 130° C.), and PVDF (Tm₂: 160° C.)), the change rate is small, i.e., less than 10%.
It can be confirmed from the results of Example 4 and Example 5 that when the Tm₁of the polymer material forming the polymer fibers is equal to or less than the glass transition point (Tg) of the polymer material forming the polymer nanofibers (Example 4: PES (Tm₁: 130° C.), PBI (Tg: 473° C.), Example 5: PES (Tm₁: 130° C.), PAI (Tg: 290° C.)), the shape of the polymer nanofibers can be maintained in the fusion-bonding process.
When the melting temperature (Melting point: Tm₁) of the polymer material forming the polymer fibers is less than Tm₂of the polymer materials forming the polymer nanofibers and the difference is large, the fusion-bonding of the polymer fibers and the polymer nanofibers coating the same can be performed by preferentially fusion-bonding the polymer fiber side.
Therefore, according to the present invention, a fiber material can be manufactured in which the polymer fibers and the polymer nanofibers coating the polymer fibers are fusion-bonded while maintaining the shape (unevenness) of the polymer nanofibers, and therefore an increase effect of the specific surface area based on the coating with the polymer nanofibers is easily demonstrated.
As shown in each Example above, according to the configuration of the present invention, a fiber material coated with nanofibers in which the peeling strength between the polymer nanofibers and the polymer fiber as the core fiber and the mechanical strength of the fiber material are high and the specific surface area is large can be provided.

INDUSTRIAL AVAILABILITY

The nanofiber coated fiber material of the present invention can be a fiber material having a large specific surface area which can be used over a long period of time even when an external factor, rubbing, is applied, and therefore can be suitably utilized, for example, as a friction charging material in a static electricity generator and a particle electric field separator.
As described above with reference to Embodiments and Examples, the present invention can provide a nanofiber coated fiber material in which the peeling resistance between nanofibers and a core fiber is excellent and the mechanical strength of the fiber material is high and a method for manufacturing the same.
While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.

Claims

What is claimed is:

1. A process for producing a fiber material, the fiber material, comprising:

an assembly of a plurality of nanofiber coated polymer fibers in which a polymer fiber serving as a core fiber is coated with polymer nanofibers,

the polymer fiber and the polymer nanofibers being bonded such that an adhesion boundary portion between the polymer fiber and the polymer nanofibers has a sheet shape or is lost,

two or more of the polymer fibers being bonded to each other in at least one portion such that an adhesion boundary portion between the two or more of the polymer fibers has a sheet shape or is lost, and

an average diameter of the polymer fiber being 10 μm to 50 μm, and an average diameter of the polymer nanofibers being 1 nm or more and less than 1 μm,

the process comprising the steps of:

(i) extruding polymer solution or melted polymer from spinning orifices simultaneously towards a collector,

(ii) forming the electro-spun polymer fiber and electro-spun polymer nanofibers,

(iii) coating the electro-spun polymer fiber with the electro-spun polymer nanofibers to form an electro-spun polymer fiber coated with the electro-spun polymer nanofibers, and

(iv) heating the electro-spun polymer fiber coated with the electro-spun polymer nanofibers, and bonding the electro-spun polymer fiber and the electro-spun polymer nanofibers each other so that an adhesion boundary portion between the electro-spun polymer fiber and the electro-spun polymer nanofibers has a sheet shape or is lost.

2. The process according to claim 1, wherein an amount of the electro-spun polymer nanofibers of the electro-spun polymer fiber coated with the electro-spun polymer nanofibers, is 5 parts by weight or more and 90 parts by weight or less.

3. The process according to claim 1, wherein temperature of the heating in the step (iv) is equal to or higher than the melting point of the polymer material forming the electro-spun polymer fiber, and is equal to or less than the glass transition point of the polymer material forming the electro-spun polymer nanofibers.