JP4656320B2 - Nanofiber, method for producing the same, and fiber product - Google Patents

Description

  The present invention relates to a nanofiber, a manufacturing method thereof, and a fiber product.

In recent years, interest in nanofibers has increased due to the expectation of function expression specific to nano-order sizes and new function expression by compounding in nano-order sizes.
Nanofibers are extremely thin fibers with a fiber diameter of nano-order, and have characteristics such as a very large specific surface area. Therefore, a filter using nanofibers not only captures finer particles but also has a large specific surface area compared to a filter using normal fibers or microfibers, so it can be used for gases, liquids and fine particles. Excellent adsorption effect.

In addition, when the fiber diameter of the fiber used for the filter becomes nano-order, the Slip Flow effect based on the interaction at the interface between the air molecule and the nanofiber surface, which is not assumed in the conventional gas theory (the effect that the air becomes slippery) ) Is expressed. Therefore, it is known that the filter using nanofibers improves the filtration efficiency under the same pressure loss condition (for example, Non-Patent Document 1).
Therefore, the nanofibers and aggregates thereof are useful for filters such as filtration filters, biochemical hazard prevention filters, cell culture carriers, catalyst carriers, fillers, lightweight clothing, young or non-woven fabric abrasives, and the like.

  Polyolefins such as polyethylene and polypropylene have (i) moderate mechanical properties and heat resistance, and (ii) high crystallinity, thus high mechanical strength and excellent chemical resistance. iii) Since it has good recyclability, it has the characteristics that it is a material suitable for the recent environmental orientation. Accordingly, nanofibers made of polyolefin are expected to be applied in various fields.

The following methods are known as methods for obtaining fibers having a very small fiber diameter.
(1) Sea-island composite spinning method (for example, Patent Document 1).
(2) Electrospinning in which a high voltage of several thousand to several tens of thousands of volts is applied between the tip of the nozzle containing the polymer solution and the substrate, and the charged polymer solution is ejected from the nozzle tip and deposited on the substrate. Law.

Examples of a method for obtaining a polyolefin fiber having a very small fiber diameter include a melt blown method and a flash spinning method (Non-Patent Document 2, Non-Patent Document 3, p. 64-67).
(3) A melt blown method in which ultrafine fibers are obtained by blowing molten polymer with a high-speed gas fluid.
(4) A flash spinning method in which a polyolefin dissolved in a solvent is discharged at a high speed to evaporate the solvent and obtain ultrafine fibers.

  The fiber diameter of the fiber obtained by the method (1) is up to about 2 μm. Therefore, nanofiber cannot be obtained by the method (1), and the fiber obtained by the method (1) is insufficient to obtain the expected effect of the nanofiber. In the method (1), it is necessary to remove the sea phase with a solvent.

  The method (2) has recently attracted particular attention as a method for producing nanofibers relatively easily. Since the solvent contained in the polymer solution evaporates before reaching the substrate, dried nanofibers can be obtained. However, since the nanofibers are (i) not oriented crystallized, the mechanical strength is very low and the chemical resistance is poor. (Ii) The obtained form can be used only for nonwoven fabrics. , Etc. have problems. In addition, the method (2) is less productive than (iii) the melt spinning method, and (iv) a high voltage is applied to the material using the organic solvent. In order to dissolve (v) polyolefin in an organic solvent, there is a problem that it is necessary to raise the temperature to 100 ° C. or higher and it is difficult to apply it to polyolefin.

The fiber diameter of the polyolefin fiber obtained by the method (3) is about 1 to 6 μm. Moreover, this fiber can be used only for a nonwoven fabric from the form when obtained.
The fiber diameter of the polyolefin fiber obtained by the method (4) is about several μm to several tens of μm. Moreover, this fiber can be used only for a nonwoven fabric from the form when obtained.
Moreover, since the polyolefin-type fiber obtained by the method of (3) and (4) is not drawn | stretched, mechanical strength is low.

The following method is disclosed as a method for producing a polyethylene fiber having high strength and high elasticity.
(5) A method of drawing after ultra high molecular weight polyethylene is discharged from a spinneret, and then drawing it after treatment with a supercritical fluid or a similar fluid (p. 81-82 of Non-Patent Document 3).

  The ultrahigh molecular weight polyethylene used in the method (5) is difficult to melt spin because the melt flow rate is so small that the melt spinning MFR cannot be measured. Therefore, ultra high molecular weight polyethylene is dissolved in a solvent to form a solution, and this solution is discharged. The gel-like undrawn yarn obtained by the so-called gel spinning method is treated with a supercritical fluid or a similar fluid. It is necessary to stretch. However, when the gel-like undrawn yarn is drawn, the fiber diameter of the obtained fiber is about several tens of μm. Therefore, the fiber obtained by the method (5) cannot be said to be a nanofiber.

As materials used for filters and the like, porous films obtained by stretching polyethylene are disclosed (Patent Documents 2 and 3). The porous film is composed of nano-order fibrils formed by stretching, but many laminated lamellae that are not fibrils remain and cannot be said to be nanofibers.
Supervised by Tatsuya Motomiya, “Advanced Industrial Discovery Strategy Using Nanofiber Technology”, CMC Publishing Co., Ltd., 2004, p. 371-373 Edited by Textile Society, “Latest Spinning Technology”, Kobunshi Publishing Co., Ltd., 1992, p. 132-126, p. 222-224 Textile Society, “Third Edition Textile Handbook”, Maruzen Co., Ltd., 2004, p. 64-67, p. 81-82 JP 2003-3323 A JP-A-57-66114 Japanese Patent No. 3168036

  Therefore, an object of the present invention is to provide a nanofiber having high mechanical strength and excellent chemical resistance, a method for producing the nanofiber, and a fiber product capable of exhibiting a specific function derived from the nanofiber.

The nanofiber of the present invention is made of a polymer material containing polyolefin, has a crystallinity measured by X-ray method of 40% or more, a fiber diameter of 300 nm or less, and a breaking strength of 100 MPa or more. And
The method for producing nanofibers of the present invention includes a step of treating and stretching a polymer material containing a polyolefin having a melt flow rate of 0.05 g / 10 min or more in a supercritical fluid or a subcritical fluid. It is characterized by that.
The temperature at which the treatment is performed in a supercritical fluid or a subcritical fluid is preferably (Tm−120) ° C. or more and (Tm + 5) ° C. or less when the melting point of the polyolefin is Tm.
The fiber product of the present invention is a fiber product having the nanofiber of the present invention.

The nanofiber of the present invention has high mechanical strength and excellent chemical resistance.
According to the method for producing nanofibers of the present invention, nanofibers having high mechanical strength and excellent chemical resistance can be obtained.
The fiber product of the present invention can exhibit a unique function derived from nanofibers.

<Nanofiber>
The nanofiber of the present invention is made of a polymer material containing polyolefin. The “nanofiber” in the present invention includes a nanofiber aggregate in which a plurality of nanofibers are aggregated, and a single-fiber nanofiber obtained by separating the aggregate. In addition, examples of the form of the nanofiber aggregate include a multifilament shape, a film shape, a sheet shape, and a hollow shape. Furthermore, when the aggregate is in the form of a film or sheet, it may be either a single layer structure or a multilayer structure.

  Examples of the polyolefin include homopolymers or copolymers of olefins such as ethylene, propylene, butene, isoprene, pentene, methylpentene, cyclohexene, norbornene, and copolymers of these olefins with vinyl acetate or vinyl alcohol. Polyethylene is preferred because it is easy to stretch. Examples of polyethylene include high density polyethylene, low density polyethylene, ethylene-vinyl acetate copolymer, ethylene-vinyl alcohol copolymer, and the like.

  The proportion of polyolefin is preferably 10% by mass or more, more preferably 50% by mass or more, still more preferably 90% by mass or more, and most preferably 100% by mass in the polymer material (100% by mass). When the proportion of the polyolefin is less than 10% by mass, the formation of the fiber structure by stretching becomes insufficient, and it may be difficult to obtain nanofibers.

  The crystallinity of the nanofiber of the present invention is 40% or more, preferably 45% or more, and more preferably 50% or more. The upper limit of crystallinity is 100%. When the crystallinity of the nanofiber of the present invention is less than 40%, the mechanical strength and chemical resistance of the nanofiber may be insufficient.

  In general, X-ray method, density method, heat of fusion method and the like are well known as methods for measuring crystallinity, but “crystallinity of nanofiber” in the present invention is the X-ray method. The crystallinity measured by is used. Here, the X-ray method is a method for obtaining the degree of crystallinity from wide-angle X-ray diffraction measurement. By utilizing the fact that crystalline diffraction and amorphous scattering can be distinguished relatively easily, the scattering intensity from the entire region is used. This is a method of measuring the scattering intensity (Ic) of the crystal region (I) and the crystal region and calculating the crystallinity from a predetermined formula [(Ic / I) × 100] (for example, p. 81 of Non-Patent Document 3). -82).

  On the other hand, the density method is a method of measuring crystallinity from the density of a sample measured in advance using the following formula (1), and is mainly used for measuring solid plastic. (This calculation formula is described in, for example, “Polymer Analysis Handbook”, first edition, pages 235 to 243 (edited by the Japan Society for Analytical Chemistry, published in 1985).)

Here, d is a measured value of density, da is a density of completely amorphous (0.856 g / cm ³ ), and dc is a density of completely crystal (1.000 g / cm ³ ).
However, in the case of the nanofiber of the present invention, extremely small voids are formed in the sample. Therefore, in a general measurement method used for measuring the density of plastic, the density may be estimated small. Therefore, the density method is not preferable as a method for measuring the crystallinity of the nanofiber in the present invention.

  On the other hand, the heat of fusion method is a method for obtaining the degree of crystallinity by a predetermined formula from the heat of fusion per unit mass of the sample obtained by differential scanning calorimetry (DSC) measurement and the heat of fusion per unit mass of crystal. . However, although the heat of fusion method is simple, there is a possibility that crystallization may proceed during the temperature rise of the DSC measurement, and this is not necessarily the numerical value of the crystallinity of the sample (nanofiber) itself that has undergone plastic deformation. Therefore, it is not preferable as a method for measuring the crystallinity in the present invention.

  The breaking strength of the nanofiber of the present invention is 100 MPa or more. Nanofibers having a breaking strength of 100 MPa or more have sufficient mechanical strength required when used as filters, nonwoven fabric abrasives, and the like. The higher the breaking strength, the better. The upper limit is not particularly limited, but it is several GPa by analogy with the breaking strength of a fiber made of ultrahigh molecular weight polyethylene having molecular chains oriented in the fiber axis direction.

  The “breaking strength” in the present invention is a stress when the nanofiber is subjected to a tensile test with a test length a (mm) and a pulling speed a (mm / min) and the breakage is reached. Although the range of the test length a depends on the nanofiber, it is preferably 5 to 200 mm, more preferably 10 to 100 mm. “Test length” is the distance between chucks when performing a tensile test. The tensile direction in the tensile test is the fiber axis direction. The tensile test is performed at room temperature (about 23 ° C.). In order to facilitate the tensile test, the nanofibers are preferably handled in an aggregated state.

  When measuring the breaking strength of nanofibers with high mechanical strength, the sample may slide in the chuck during the tensile test, and the apparent stress may decrease before the sample breaks (apparent yield stress). The breaking strength in this case is higher than the apparent yield stress. In order to prevent chuck slippage, the sample may be chucked through sandpaper.

In the case of a nanofiber aggregate in which a plurality of nanofibers are aggregated, the fiber diameter of the nanofiber in the present invention means an average of the fiber diameters of the single fibers constituting the aggregate, In the case, it means the fiber diameter of one nanofiber.
The fiber diameter of the nanofiber of the present invention is 300 nm or less, preferably 200 nm or less, and more preferably 150 nm or less. When the fiber diameter is larger than 300 nm, the effect as a nanofiber may be insufficient. Although the lower limit of the fiber diameter is not particularly provided, it is usually 1 nm or more.

The fiber diameter of the nanofiber of the present invention is measured as follows.
First, the surface of the nanofiber is observed using an electron microscope. When using a nanofiber aggregate as a sample, the nanofiber aggregate may be frozen and cleaved using liquid nitrogen or the like, and the surface of the nanofiber that has appeared may be observed. By this method, for example, as shown in FIG. 1, the surface of the single fiber constituting the nanofiber aggregate can be observed.
As the electron microscope, a scanning electron microscope (SEM) is preferable. The observation magnification is about 10,000 times to 100,000 times. When the observation magnification is less than 10,000, it is difficult to determine the fiber diameter of the nanofiber. Therefore, in order to obtain the fiber diameter, it is preferable to use a field emission SEM.

  Next, the fiber diameter of the nanofiber is obtained from the obtained SEM photograph. The fiber diameter was measured by measuring the width (diameter) in the direction perpendicular to the longitudinal direction of any 20 nanofibers in the SEM photograph, and then the average value (average fiber diameter) of the fiber diameters of the 20 nanofibers. This is the fiber diameter of the nanofiber. When obtaining the fiber diameter, it is preferable to use image analysis software. The fiber diameter obtained by the image analysis software varies slightly depending on the image quality adjustment for image analysis, the type of image analysis software, and the like, but the difference is within the range of normal experimental error.

The fiber diameter of the nanofiber of the present invention is determined as follows in the case of a single fiber.
A surface of one nanofiber or a longitudinal section perpendicular to the fiber axis direction is observed using an electron microscope. Next, the fiber diameter of the nanofiber is obtained from the obtained SEM photograph. For the fiber diameter, 20 widths (diameters) in the direction perpendicular to the longitudinal direction of the fiber on the nanofiber surface are measured, and this is defined as the fiber diameter of the nanofiber.

When observing nanofibers with an SEM, if only one single-fiber nanofiber can be sampled, the method described above may be used, but if it is difficult to handle only one, a nanofiber assembly You may perform SEM observation about. In this case, the fiber diameter of each nanofiber is obtained, and the average value of these is taken as the fiber diameter of the nanofiber.
In order to measure the fiber diameter using the nanofiber aggregate, select any 20 of the nanofiber aggregates in the SEM photograph, measure the width near the center of each, and calculate the average value of the nanofibers. The fiber diameter. In addition, when 20 or more nanofibers cannot be imaged in the SEM photograph, it is only necessary to measure 20 places in the entire photograph by measuring two or more different places in the same fiber.

  The form of the nanofiber aggregate used for the measurement is not particularly limited, and may be any of a fiber shape, a film shape, a sheet shape, and a hollow shape.

<Manufacturing method of nanofiber>
The method for producing nanofibers of the present invention is a method in which a polymer material containing polyolefin having a melt flow rate of 0.05 g / 10 min or more is exposed to a supercritical fluid or a fluid in a subcritical state, and is stretched. It is. That is, the stretching of the polymer material may be performed in (i) a supercritical fluid or a subcritical fluid, and (ii) after exposure to the supercritical fluid or subcritical fluid, outside the fluid. You may go. Further, the film may be preliminarily stretched before being exposed to a supercritical fluid or a subcritical fluid.

  The “supercritical fluid” in the present invention refers to a high-density fluid that does not condense even when a temperature and pressure above the critical point are applied. This state has the same diffusivity as a gas while having the same density as a liquid. Therefore, the supercritical fluid is a fluid that penetrates into the details of the polymer material and has a large plasticizing effect.

  The “subcritical fluid” in the present invention is a fluid having a temperature or pressure higher than the critical point, and the high-pressure fluid penetrates to the inside of the polymer material as in the supercritical fluid. Has the effect of plasticizing.

  As the supercritical fluid or subcritical fluid, carbon dioxide (critical temperature: 31.0 ° C., critical pressure: 7.38 MPa) and nitrous oxide (critical temperature: 36. 3) at a temperature and / or pressure above the critical point. 5 ° C., critical pressure 7.27 MPa), ethane (critical temperature 32.2 ° C., critical pressure 4.88 MPa), ethylene (critical temperature 9.34 ° C., critical pressure 5.04 MPa), and the like.

  Carbon dioxide is preferred as the main component of the supercritical fluid or subcritical fluid. Since carbon dioxide has a critical temperature of 31.0 ° C. and a critical pressure of 7.38 MPa, it is relatively easy to handle, nonflammable, inert, non-toxic, inexpensive, and suitable for supercritical conditions. . In the present invention, "the main component of the supercritical fluid or subcritical fluid is carbon dioxide" means that the proportion of carbon dioxide in the supercritical fluid or subcritical fluid (100% by volume) is 80 to 100 volumes. Means%.

  Examples of the polyolefin used in the production method of the present invention include polyolefins exemplified as the material of the nanofiber of the present invention. As the polyolefin, polyethylene is preferable because it can be easily stretched and nanofibers can be easily obtained. Examples of the polyethylene include high density polyethylene and low density polyethylene.

The crystallinity of the raw material polyolefin used for producing the nanofiber is 40% or more, preferably 45% or more, and more preferably 50% or more. If the crystallinity of the polyolefin is less than 40%, the crystallinity of the obtained nanofiber may not be 40% or more, or it may be difficult to obtain the nanofiber itself.
The “crystallinity of the raw material polyolefin” in the present invention is a value measured using a density method. This is because the polymer material used in the production of the nanofiber of the present invention is a densely packed material such as a pellet, unlike the case of the previous nanofiber. It is possible to measure the crystallinity at Conversely, when the X-ray method is used, it is necessary to shape the fiber or film for X-ray measurement, and the crystallinity may change due to the thermal history received at that time. Further, in the heat of fusion method, crystallization may proceed during the temperature rise of DSC measurement. Therefore, each method may not be able to accurately measure the crystallinity of the raw material polyolefin.

  The melt flow rate (MFR) of the raw material polyolefin is 0.05 g / 10 min or more, preferably 0.1 g / 10 min or more, more preferably 0.2 g / 10 min or more. Moreover, 10 g / 10min or less is preferable. By setting the melt flow rate of polyolefin to 0.05 g / 10 min or more, the melt shapeability becomes high and the mobility of the molecular chain of polyolefin is sufficiently improved. As a result, the fiber structure can be formed by stretching. It becomes. The melt flow rate is a value measured according to JIS K7210. The MFR tends to decrease as the number average molecular weight of the raw material polyolefin increases. Therefore, an ultra-high molecular weight polyolefin that is less than 0.05 g / 10 min is not suitable for producing the nanofiber of the present invention.

  In the production method of the present invention, the proportion of the polyolefin having a crystallinity of 40% or more is preferably 10% by mass or more, more preferably 50% by mass or more, and 90% by mass or more in the polymer material (100% by mass). More preferred is 100% by mass. When the proportion of the polyolefin having a crystallinity of 40% or more is less than 10% by mass, the fiber structure may not be sufficiently formed by stretching.

  The temperature at which the polymer material is treated by exposing it to a supercritical fluid or subcritical fluid (hereinafter referred to as the treatment temperature) is (Tm−120 ° C.) or more (Tm + 5), where Tm is the melting point of the polyolefin. ° C) or less, preferably (Tm-60 ° C) or more and (Tm + 5 ° C) or less. If the treatment temperature is less than (Tm-120 ° C.) ° C., the mobility of the polyolefin molecular chain having a crystallinity of 40% or more is insufficiently improved during treatment in a supercritical fluid or subcritical fluid. Therefore, there is a fear that the fiber structure is not sufficiently formed by stretching. When the treatment temperature is higher than (Tm + 5 ° C.) ° C., the polymer material is melted, and it may be difficult to perform the stretching operation in a supercritical fluid or a subcritical fluid or after the treatment. When the polyolefin is polyethylene, the treatment temperature is preferably 20 ° C. or higher and 140 ° C. or lower, and more preferably 80 ° C. or higher and 140 ° C. or lower. In addition, melting | fusing point Tm of polyolefin is determined based on the measuring method prescribed | regulated by JISK7121.

  The pressure (hereinafter referred to as processing pressure) at the time of processing by exposing the polymer material to a supercritical fluid or subcritical fluid is preferably 2 MPa or more, and more preferably 3 MPa or more. If the treatment pressure is less than 2 MPa, the supercritical fluid or subcritical fluid may not sufficiently penetrate into the polymer material. The upper limit of pressure is limited only by the pressure resistance condition of the apparatus.

  The treatment time (hereinafter referred to as treatment time) by exposing the polymer material to the supercritical fluid or subcritical fluid is preferably 5 minutes or more and 5 hours or less, more preferably 15 minutes or more and 1 hour or less. If the treatment time is less than 5 minutes, the supercritical fluid or subcritical fluid may not sufficiently penetrate into the polymer material. Moreover, even if it processes for 5 hours or more, there is hardly any change in the permeation state of the supercritical fluid or the subcritical fluid, which tends to be disadvantageous in terms of productivity and cost of the nanofiber.

The shape of the polymer material (precursor) before the stretching operation is not particularly limited, and examples thereof include a film shape, a sheet shape, and a hollow shape. Moreover, as a shaping method of a precursor, well-known methods, such as sintering shaping | molding, melt shaping, and wet shaping, can be used.
Moreover, in the manufacturing method of this invention, you may use an entrainer (addition solvent) in any of the above-mentioned method (i) and method (ii).

The nanofiber formation mechanism in the production method of the present invention is not clear, but is considered as follows.
When the supercritical fluid or subcritical fluid penetrates into the polymer material, the mobility of the molecular chain of the polyolefin having a crystallinity of 40% or more in the polymer material becomes active. Alignment is likely to occur, for example, a complete crystal structure with fewer tie molecules is formed. When such a polymer material is stretched, there is little entanglement of the stretched molecular chains, so that the unit of fibrils formed by stretching becomes small, and nano-order fibrils, that is, nanofiber aggregates are formed.

  The nanofiber aggregates thus formed can be used as they are for the secondary processing for the production of filters and catalyst carriers. By separating the aggregates, the single-fiber nanofiber aggregates can be used. You can also get fiber. Examples of the fiber separation method include a method of immersing the nanofiber aggregate in a solvent and applying ultrasonic waves thereto.

<Textile products>
The fiber product of the present invention is a fiber product having the nanofiber of the present invention. The form of the nanofiber of the present invention used for the fiber product may be a nanofiber assembly, or a single-fiber nanofiber obtained by splitting the nanofiber.
Examples of the fiber product of the present invention include filters such as filtration filters and biochemical hazard prevention filters; cell culture carriers, catalyst carriers, fillers, lightweight clothing, nonwoven fabric abrasives, and the like.

Hereinafter, the present invention will be described in more detail with reference to examples. The present invention is not limited to these examples.
The evaluation method in the present example is as follows.

(Observation of nanofibers)
The surface of the nanofiber was observed using a field emission scanning electron microscope JSM-7400F manufactured by JEOL Ltd. (SEM observation). The surface of the nanofiber was observed by freezing and cleaving the nanofiber in liquid nitrogen in parallel to the longitudinal direction of the nanofiber, and observing the surface of the nanofiber that appeared.

(Fiber diameter of nanofiber)
The fiber diameter of the nanofiber was determined by image analysis of the SEM photograph using image analysis software (Image-Pro Plus ver4.5.0.24 manufactured by Planetron Co., Ltd.). After adjusting the scale of the image analysis to match the scale of the SEM photograph, the surface width of any 20 nanofibers in the SEM photograph was determined, and the average value was taken as the fiber diameter of the nanofibers.
(MFR measurement of raw polyethylene)
According to JIS K7210, the measurement was performed under the conditions of a test temperature of 190 ° C. and a test load of 2.16 kg.

(Measurement of crystallinity of raw material polyolefin by density method)
The crystallinity was calculated from the density (catalog value) of the polyethylene used using Equation (1).
(Measurement of crystallinity of nanofibers by X-ray method)
The wide-angle X-ray scattering measurement was performed at an output of 40 kV-100 mA using a RU-200 manufactured by Rigaku Corporation with the sample stretching direction fixed vertically. Next, the crystallinity was calculated using the diffraction peaks of [110] and [200] using crystallinity analysis software (multiple peak separation method).

(Measurement of breaking strength of nanofiber)
The breaking strength of the nanofiber was measured using a tensile tester (manufactured by Orientec Co., Ltd., UCT-500) at a chuck distance of 20 mm, a pulling speed of 20 mm / min, and a temperature of 23 ° C.

[Example 1]
As a polyolefin having a crystallinity of 40% or more, Mitsui Chemicals, high density polyethylene, Hi-Zex 2200J (melt flow rate = 5.2 g / 10 min (catalog value), density = 964 kg / (crystal obtained from density) Degree of conversion = 78%) and Tm = 135 ° C.). This high-density polyethylene was hot-pressed at 180 ° C. to produce a film having a thickness of 180 μm. A strip sample having a width of 5 mm and a length of 40 mm was cut from the film. This was put in a pressure vessel and the inside of the vessel was replaced with carbon dioxide. The strip-shaped sample was treated in supercritical carbon dioxide for 1 hour at a temperature inside the container of 125 ° C. and a pressure of 10 MPa. After completion of the treatment, the inside of the container was depressurized to atmospheric pressure, and the film was recovered. The collected film was turned around and attached to a stretcher, and stretched 14 times in a hot bath at 80 ° C. to obtain a stretched sample (nanofiber aggregate). FIG. 1 shows an SEM photograph in which the obtained nanofiber aggregate was frozen and cut parallel to the longitudinal direction, and the surface of the nanofiber appeared. Table 1 shows the average fiber diameter, breaking strength, and crystallinity. A nanofiber assembly having an extremely small average fiber diameter and sufficient breaking strength and crystallinity was formed. When measuring the breaking strength of the obtained sample, chuck slip occurred before breaking, so the maximum stress was 266 MPa, but the actual breaking strength is 266 MPa or more.

[Comparative Example 1]
A stretched sample was obtained in the same manner as in Example 1 except that the strip sample was heat-treated at 125 ° C. under atmospheric pressure instead of being treated in supercritical carbon dioxide. FIG. 2 shows an SEM photograph in which the obtained stretched sample was frozen and cut parallel to the drawing direction, and the surface of the appearing fiber was observed. Table 1 shows the average fiber diameter, breaking strength, and crystallinity. A fiber assembly having an average fiber diameter exceeding 300 nm was obtained. In addition, although the SEM photograph (FIG. 1) of Example 1 was observation at 30,000 times, when the stretched sample of Comparative Example 1 was observed at an observation magnification of 30,000, the fiber structure was large, and thus the average fiber diameter was obtained. Therefore, the average fiber diameter was determined with an observation magnification of 10,000.

  The nanofiber of the present invention combines chemical resistance and mechanical strength, and is useful for filtration filters, biochemical hazard prevention filters, cell culture carriers, catalyst carriers, fillers, lightweight clothing, nonwoven fabric abrasives, etc. .

2 is a longitudinal cross-sectional SEM observation photograph of the nanofiber aggregate obtained in Example 1. FIG. 4 is a longitudinal cross-sectional SEM observation photograph of a stretched sample obtained in Comparative Example 1.

Claims (4)

  A nanofiber comprising a polymer material containing polyolefin, having a crystallinity of 40% or more measured by an X-ray method, a fiber diameter of 300 nm or less, and a breaking strength of 100 MPa or more.   A method for producing nanofibers, comprising a step of treating and stretching a polymer material containing a polyolefin having a melt flow rate of 0.05 g / 10 min or more in a supercritical fluid or a subcritical fluid.   The nanofiber according to claim 2, wherein the temperature at the time of processing in the supercritical fluid or subcritical fluid is (Tm-120) ° C or higher and (Tm + 5) ° C or lower when the melting point of the polyolefin is Tm. Manufacturing method. A textile product comprising the nanofiber according to claim 1.

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