JP2006265788A - Method for producing conjugated fiber - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a conjugated fiber, by which the conjugated fiber suitable for producing ultra fine carbon fibers can efficiently be produced. <P>SOLUTION: This method for producing the conjugated fiber is characterized by producing the sea-island conjugated fiber comprising a carbon raw material resin having melt shapability as the island component and a thermally decomposable resin as a sea component by a melt-spinning method so that the rate of the island component resin occupying in all of the fiber is 10 to 50 wt.% and the diameter of the island component is 1 to 10 μm, and then super-drawing the obtained fiber. It is preferable that the island component and the sea component are a melt-shapable acrylonitrile-based resin and an aliphatic polyester, respectively. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method for producing a composite fiber useful for producing fine carbon fibers.

  The characteristic value represented by the strength of carbon fiber depends greatly on the number of defects in the fiber, and the number of defects increases with the thickness of the fiber. It is possible to greatly improve the performance of the fiber. Carbon fibers are often impregnated into a resin or the like and used as a reinforcing agent. In that case, in addition to the strength of the carbon fibers themselves, the adhesion to the resin greatly affects the performance of the composite material. Here too, when the carbon fiber is thinned, the surface area per unit weight increases, so that the adhesion can be improved and the performance of the composite material can be greatly improved.

  Carbon nanotubes that have become a hot topic in recent years are materials called ideal carbon fibers formed from almost perfect graphite sheets, and their physical properties are much higher than those of ordinary carbon fibers. In particular, single-walled nanotubes are very thin with a diameter of 10 nm or less, and are highly expected as additives for resins and the like. However, since carbon nanotubes are manufactured by a bottom-up method in which carbon atoms are stacked one by one, the productivity is very low and the price is high. For this reason, although it has excellent performance, it has not been widely used. Therefore, many attempts have been made to improve the production method of a normal carbon fiber with high productivity to produce carbon fibers that are as thin as possible, that is, carbon nanofibers, and obtain performance close to that of carbon nanotubes.

  In recent years, blended fibers are created by mixing a resin that forms carbon fibers by firing (hereinafter referred to as carbon raw material resin) and a resin that decomposes and burns down during firing (hereinafter referred to as thermally decomposable resin). However, a method for obtaining fine carbon fibers by firing the same has been developed. For example, Japanese Patent Laid-Open Nos. 2001-73226 and 2003-20517 use a phenol resin as a carbon raw material resin, and perform blend spinning with polyethylene (PE) and polypropylene (PP), which are thermally decomposable resins. A method for obtaining fine carbon fibers is described.

  If a method using such blend spinning is used, it is possible to obtain a form in which a carbon raw material resin is dispersed in a thermally decomposable resin relatively easily, and fine carbon fibers are obtained by firing this. It is also possible. However, when blend spinning is used, the size of the island component is determined by the compatibility between the resins and the kneading state, so that there is a problem that it is difficult to control the size of the island component to an arbitrary size. In addition, because the island components that have become spherical during kneading are fiberized by stretching in the fiber axis direction during spinning, the resulting carbon fibers are not continuous fibers but a collection of short fibers with a certain length Therefore, there is a problem that it cannot be used as a reinforcing material for long fiber reinforced materials that are often used as structural materials for aircraft and the like.

  On the other hand, attempts have been made to produce fine carbon raw material resin fibers using the technology of sea-island type composite fibers used when producing ultrafine synthetic fibers. For example, Patent Document 1 describes a method for producing a composite fiber that is melt-spun using a spinneret apparatus using an acrylonitrile-based polymer that can be melt-formed as a carbon precursor and acid-modified PE or methyl methacrylate (PMMA) as a sea part. Has been. However, in the composite fiber that can be produced by this method, the diameter of the island component is generally about 5 to 10 μm, and it is very difficult to produce a fiber having an island component diameter of 1 μm or less. It could not be produced.

  Patent Document 2 discloses a method for producing a carbon nanofiber by firing a sea-island composite fiber in order to produce a carbon nanofiber having a long length, and a method for obtaining the sea-island composite fiber. Describes a method in which fibers made of a carbon raw material resin and fibers made of a heat-decomposable resin are bundled and then extruded from the pores and super-stretched. However, it is difficult to mix different kinds of fibers uniformly, and it is very laborious, so productivity is poor. Furthermore, in order to super stretch the sea-island composite fiber, it is necessary to improve the fluidity by heating the composite fiber at a high temperature, but the sea-island structure is likely to be disturbed due to the difference in the flow characteristics of the amphoteric during heating, It was difficult to obtain uniform fine carbon fibers due to coalescence of island components and breakage of island components. In particular, when the island component is refined to the nano level, the sea-island structure is destroyed even by a slight turbulence in the flow, so that it is very difficult to obtain carbon fibers having a nano level fineness.

As described above, despite the demand for fine and continuous carbon fibers, there has been no method for stably producing a composite fiber composed of a carbon raw material resin and a thermally decomposable resin.
JP 2004-43994 A JP 2003-301335 A

  An object of this invention is to provide the method of producing efficiently the composite fiber suitable for manufacturing a fine carbon fiber.

  The above-mentioned purpose is to produce a sea-island composite fiber using a carbon raw material resin having melt-forming properties as an island component and a thermally decomposable resin as a sea component by melt spinning so that the island component has a diameter of 1 to 10 μm, Furthermore, it can achieve by the manufacturing method of the composite fiber characterized by super-drawing the obtained fiber.

  In the production of carbon fiber raw material fiber containing carbon raw material resin in the thermally decomposable resin, sea island fiber whose shape of island component is controlled by melt spinning with excellent productivity is created, and this is super-stretched to make island component It is possible to stably produce a composite fiber having a continuous island component made of a carbon raw material resin.

In the method for producing a composite fiber of the present invention, it is important to first create a sea-island composite fiber having an island component of a carbon raw material resin having melt-forming properties and a sea component of a thermally decomposable resin. In general, the carbon raw material resin is poor in spinning performance, and when spinning alone, it is difficult to obtain fibers having a diameter of 20 μm (about 5 dtex) or less from the viewpoint of stability. However, by using a composite fiber with a thermally decomposable resin excellent in yarn forming property, it becomes possible to improve the yarn forming property and to refine the fiber. Furthermore, by using a sea-island type composite fiber having a plurality of island components in one fiber, the thickness of the island component relative to the thickness of the composite fiber can be remarkably reduced, and the diameter is 10 μm (about 1 dtex). It is possible to stably produce a composite fiber having the following island components. In order to obtain a fine carbon fiber, it is desirable to make the island component in the composite fiber fine at the time of melt spinning, and it is more preferable that the diameter of the island component is 5 μm or less. However, if the island component is made too fine during melt spinning, the amount of resin supplied to one island will be very small, resulting in large variations in fineness among island components, and partial island component rupture in the subsequent drawing process. Therefore, the diameter of the island component in the composite fiber during melt spinning is preferably 1 μm or more, and more preferably 2 μm or more. Such a sea-island composite fiber can be produced by performing melt spinning using a spinneret as described in Japanese Patent Publication No. 47-26723, for example.
The carbon raw material resin in the present invention refers to a resin having an ability to become a carbon fiber by performing a baking treatment after performing an appropriate pretreatment, specifically, after performing a predetermined pretreatment, The heat loss rate after heating at 10 ° C./min from room temperature to 600 ° C. in a nitrogen atmosphere using a thermogravimetric analyzer and then holding for 1 hour is 80% or less. Typical examples of such resins include phenolic resin, pitch, polyacrylonitrile, cellulose, etc. In the present invention, it is necessary to have thermoplasticity because melt spinning is used as the spinning method, and a novolak type Polyacrylonitrile resin (hereinafter referred to as thermoplastic polyacrylonitrile resin) obtained by copolymerizing a phenol resin, butadiene, methyl acrylate, or the like to impart melt-forming properties is preferred.
The thermally decomposable resin refers to a resin that is completely thermally decomposed by heat treatment at a high temperature. Specifically, the resin is removed from room temperature to 600 ° C. under nitrogen using a thermogravimetric analyzer at 10 ° C. / When the temperature is raised in minutes, the weight reduction rate is over 95%. Examples of such resins include polyolefins such as polyethylene and polypropylene, aliphatic polyamides such as nylon 6, nylon 6,6, and aliphatic polyesters such as polylactic acid.

  In the method for producing a composite fiber of the present invention, it is important to refine the island components in the composite fiber by super-drawing the sea-island type composite fiber obtained by melt spinning. By miniaturizing the island component by super-drawing, the diameter of the carbon fiber obtained after firing the composite fiber becomes fine, and it becomes possible to obtain a carbon fiber having excellent characteristics. Here, in the method of super-drawing single component fibers, the original fiber is thick, so that super-stretching at a very high magnification is required, and therefore productivity is lowered. On the other hand, if a blend fiber having a small diameter of the island component before stretching is used, the draw ratio can be kept low, but the variation in the diameter of the carbon fibers becomes large, and continuous carbon fibers cannot be obtained. As in the present invention, it is possible to produce a continuous carbon nanofiber efficiently by keeping the draw ratio low by performing super-drawing with a sea-island type composite fiber that can create a uniform and fine sea-island structure. It becomes possible.

  Usually, the fiber is stretched by heating the fiber to a temperature slightly higher than the deformation temperature using a heating roll and winding the fiber with a winding roll set at a higher speed to make the fiber thinner. At this time, the fibers are drawn and thinned, and at the same time, the molecular orientation advances and the strength and elongation characteristics are improved. However, since the stretching tension increases with the progress of molecular orientation and leads to fiber breakage, it is impossible to stretch at a certain stretching ratio. In general, the limit is 5 to 7 times for polyester and polyamide, and the limit is 15 to 20 times for polyethylene and polypropylene that can easily increase the draw ratio. On the other hand, super-stretching is higher than normal stretching by suppressing the increase in stretching tension by stretching the fiber in a state where the molecular orientation does not advance sufficiently by heating the fiber to a high temperature close to the melting point. This refers to stretching performed at a magnification, and in the present invention refers to stretching at a stretching ratio of 20 times or more.

  In super-stretching, the fiber heating method is important. However, in order to heat the fiber to a semi-molten state, a non-contact heating method such as laser beam or steam is preferable. Among these, a laser beam is preferable because the fiber can be rapidly heated in a narrow range, and thus superstretching is easy to stabilize. Even when a contact heater is used, the shape of the fiber is such that the fiber inlet 2 is tapered as shown in FIG. 1 and is drawn simultaneously with heating using a tapered die 1 leading to the fiber outlet 3. Stable stretching can be achieved by devising a technique that does not collapse. At this time, if the draw ratio is high, the stretched fiber becomes extremely thin and the stretched yarn is easily broken. Therefore, it is preferable to bundle a plurality of fibers in advance to increase the thickness of the stretched fiber and stabilize the stretching.

  In this way, in order to produce a composite fiber with fine island components by combining sea-island composite spinning and ultra-drawing, it prevents island components from rupturing and coalescence between island components during refinement by spinning and drawing. There is a need. For this reason, in the composite fiber of this invention, it is important that the ratio of the island component with respect to the mass of the whole composite fiber is 10 to 50% by weight. By setting it as such a fraction, it is possible to prevent the island components from being united with each other in the spinning / drawing step, and to efficiently miniaturize the island components. In particular, in the sea-island composite fiber, there is a tendency for the sea component resin to decrease in the outer peripheral portion. May be united. In order to prevent such union of island components, the weight fraction of the island components is preferably 45% or less, and more preferably 40% or less. In order to increase the yield of carbon fiber after firing, the weight fraction of the island component is preferably 15% or more, and more preferably 25% or more.

  Furthermore, if the flow characteristics of the sea component resin and the island component resin are different, the island components may be united or the thickness of the island components may be uneven due to the uneven pressure balance inside the spinneret. In addition, if the flow characteristics are greatly different, the flow is disturbed during super-stretching, causing island components to break or coalesce. For this reason, it is preferable that the flow characteristics of the sea component resin and the island component resin at the time of super-stretching are similar, and it is particularly preferable that the viscosity ratio of the sea component resin and the island component resin is within a certain range. This viscosity ratio is represented by the following formula.

η _s : Viscosity of sea component (thermally decomposable resin) η _i : Viscosity of island component (carbon raw material resin) Here, each resin component has a higher melting point so that both resins can be molded. Measured at a temperature 50 ° C. higher than the melting point of the resin. Here, the melting point in the present invention is an endotherm observed when a resin to be measured is measured from a room temperature under a temperature rising condition of 16 ° C./min in a differential calorimetry performed by a differential scanning calorimeter (DSC manufactured by Perkin Elmer). Refers to peak temperature. Here, for a resin that does not show crystallinity and does not show a clear endothermic peak in measurement with a differential scanning calorimeter, the softening point is used instead of the melting point. The melt viscosity is a value measured with a capillary rheometer (Toyo Seiki Seisakusho Co., Ltd. Capillograph Type 1B).

  By using the composite fiber thus obtained, fine carbon is obtained by performing a process of imparting thermal stability to the carbon raw material resin in the composite fiber and a process of firing the composite fiber to produce carbon fiber. Fiber can be produced.

  Here, in order for the carbon fiber obtained after firing the composite fiber to exhibit sufficient performance as a carbon nanofiber, the average diameter of the island component formed by the carbon precursor in the cross section perpendicular to the fiber axis is It is preferable to reduce the size to 500 nm or less. The diameter of the island component in the cross section shown in the present invention can be measured by staining a thin sample cut in the direction perpendicular to the fiber axis as necessary and observing with a transmission electron microscope (TEM), Image processing is performed on a TEM photograph containing an area of 1/10 or more of the fiber cross section, the diameter of each island component is calculated, and the average diameter can be obtained from the diameter of the obtained island component. Moreover, since the performance improves, so that carbon nanofiber is thin, the average diameter of an island component is more preferable if it is 200 nm or less, and it is further more preferable if it is 100 nm or less. However, if it becomes too thin, it is predicted that the formability of the graphite sheet will be adversely affected. Therefore, the diameter is preferably 1 nm or more.

  A fine carbon fiber can be obtained by imparting thermal stability to the composite fiber obtained as described above and firing it. The step of imparting thermal stability to the carbon raw material resin in the composite fiber is a step of imparting thermal stability so that the shape of the carbon raw material resin that had melt-shaped property during spinning does not change during firing. . The heat stability may be imparted by a method suitable for the carbon raw material resin. For example, in the case of a phenol resin, the phenol resin can be cured by aldehydes in the presence of a crosslinking catalyst. Moreover, in polyacrylonitrile resin, it is possible to impart thermal stability by cyclizing acrylonitrile groups by prolonged heating or chemical treatment.

  In the process of firing the composite fiber to obtain the carbon fiber, elements other than carbon inside the carbonized resin are removed to produce a fiber made of pure carbon. Here, since the carbon raw material resin is carbonized and the thermally decomposable resin is completely removed, ultrafine carbon fibers derived from the island raw material carbon raw material resin can be obtained. Firing may be performed according to a known method, and it is preferable to perform heat treatment at 600 to 1200 ° C. under an inert gas such as nitrogen or argon.

  The carbon fibers obtained in this way are characterized by being extremely thin reflecting the shape of the carbonized resin in the composite fiber of the present invention, and can be used for high-performance electrodes and adsorbents utilizing a large surface area. In addition, it is also useful as a resin additive because of its excellent mechanical properties. In particular, the carbon fiber obtained from the composite fiber of the present invention is obtained in a completely continuous form, unlike the ultrafine carbon fiber obtained from the conventional gas phase method or blend fiber, and is particularly useful as a reinforcing material for long fiber reinforced composite materials. Useful.

Hereinafter, the present invention will be described more specifically with reference to examples. In addition, the various measured values in each Example are the values measured by the following method.
A. Total fineness of undrawn yarn, single yarn fineness Fiber of 100 m length was collected from a winding drum using a measuring machine, and the total fineness of the undrawn yarn was determined by multiplying the measured weight by 100. The single yarn fineness was determined by dividing the total fineness by the number of filaments.
B. Diameter of island component in unstretched yarn Cut the fiber through a copper plate and cut with a razor to prepare a sample with a thickness of 0.5 mm in the direction perpendicular to the fiber axis, and observe the cross section using an optical microscope (Olympus Corporation BX60) The diameter of the island component was obtained from the observed image.
C. Circularity and average diameter of island component in drawn fiber First, after impregnating the drawn fiber with epoxy resin and dyeing with RuO _4, an ultrathin section with a plane perpendicular to the fiber axis is prepared using a microtome and transmitted. The center part of the fiber was observed at a magnification of 40,000 times with a scanning electron microscope (Hitachi Ltd., H-7100FA). Next, the obtained image was subjected to image processing with image processing software (Winroof, manufactured by Mitani Corporation), and the area and circularity of each island component were obtained. Furthermore, the diameter in terms of a circle was calculated from the area of each island component obtained, and the average diameter was calculated from the following formula.

D. Melting point Measurement was performed using a differential scanning calorimeter (DSC manufactured by PerkinElmer Co., Ltd.) at a temperature rising condition of 16 ° C./min from room temperature, and the observed endothermic peak temperature was taken as the melting point.
E. Melt Viscosity Using a capillary rheometer (Capillograph 1B type, manufactured by Toyo Seiki Seisakusho Co., Ltd.), the viscosity was measured at a shear rate of 1216 sec ⁻¹ using a die having a pore diameter of 1 mmφ and L / D = 10.

Example 1
Sea-island type composite fibers were produced under the following conditions.
Island component: Thermoplastic polyacrylonitrile resin (Mitsui Chemicals, Valex (registered trademark) # 3000)
Sea component: Polylactide (Cargill Dow Polymer LLC, 6200D, melting point 170 ° C., melt viscosity 155 Pa · sec at 220 ° C.)
Spinneret: 36 islands × 16 filaments Sea-island ratio: island / sea = 30/70 (weight ratio)
Total discharge amount: 25 g / min
Spinning temperature: 220 ° C
Winding speed: 1300m / min
The spinnability at the time of spinning was good, and the obtained undrawn yarn had a total fineness of 192 dTex and a single yarn fineness of 12 dTex. Further, when the cross-section of the undrawn yarn was observed, the island component was uniformly observed inside the fiber, and the diameter thereof was 3.3 μm. 550 fibers were drawn and filled in a heat-shrinkable tube, treated in a vacuum dryer at 120 ° C. for 3 hours to fuse the fibers together, and then shaped to produce a rod having a diameter of 10 mm. The obtained rod was pressed into a tapered die heated to 210 ° C. having a diameter of 10 mm on the inlet side and a diameter of 1 mm on the outlet side, and the discharged fiber was taken out to obtain a monofilament having a diameter of 0.73 mm. The theoretical stretching ratio calculated from the diameter before and after stretching is 193 times.

    As a result of cross-sectional observation of the obtained monofilament, it was a sea-island type composite fiber containing countless island components made of thermoplastic polyacrylonitrile inside, and the island components were almost completely independent. Here, the average diameter of the island component was 234 nm, and the ratio of the island component having a circularity of 0.9 or more was 96.3%.

  This sea-island type composite fiber was heat treated at 240 ° C. for 45 minutes in a nitrogen atmosphere to make it infusible, and then subjected to heat treatment at 600 ° C. for 1 hour in a nitrogen atmosphere to produce a carbon fiber. When the produced carbon fiber was observed with an electron microscope, a bundle of continuous carbon fibers having a diameter of 180 nm was observed.

Example 2
Sea-island type composite fibers were produced under the following conditions.
Island component: Thermoplastic polyacrylonitrile resin (Mitsui Chemicals, Valex (registered trademark) # 3000)
Sea component: Polylactide (Cargill Dow Polymer LLC, 6400D, melting point 170 ° C., melt viscosity 265 Pa · sec at 220 ° C.)
Spinneret: 36 islands × 16 filaments Sea-island ratio: island / sea = 40/60 (weight ratio)
Total discharge amount: 25 g / min
Spinning temperature: 220 ° C
Winding speed: 1300m / min
The spinnability at the time of spinning was good, and the obtained undrawn yarn had a total fineness of 192 dTex and a single yarn fineness of 12 dTex. Further, when the cross section of the undrawn yarn was observed, the island component was uniformly observed inside the fiber and the diameter was 3.8 μm. This fiber was ultradrawn in the same manner as in Example 1 to produce a monofilament having a diameter of 0.73 mm.

    As a result of cross-sectional observation of the obtained monofilament, it is a sea-island type composite fiber containing countless island components made of thermoplastic polyacrylonitrile inside, and most of the island components are independent, but the distance between the island components is It was observed that some island components were fused because they were close. Here, the average diameter of the island component was 284 nm, and the proportion of islands having a circularity of 0.9 or more was 84.5%.

Example 3
Sea-island type composite fibers were produced under the following conditions.
Island component: Thermoplastic polyacrylonitrile resin (Mitsui Chemicals, Barex (registered trademark) # 3000)
Sea component: Polylactide (Cargill Dow Polymer LLC, 6250D, melting point 170 ° C., melt viscosity at 220 ° C. 83 Pa · sec)
Spinneret: 36 islands × 16 filaments Sea-island ratio: island / sea = 20/80 (weight ratio)
Total discharge amount: 25 g / min
Spinning temperature: 220 ° C
Winding speed: 1300m / min
The spinnability at the time of spinning was good, and the resulting undrawn yarn had a total fineness of 192 dTex and a single yarn fineness of 12 dTex. Further, when the cross-section of the undrawn yarn was observed, the island component was uniformly observed inside the fiber and the diameter was 2.7 μm. 3,500 fibers were drawn and filled in a heat-shrinkable tube, treated in a vacuum dryer at 120 ° C. for 3 hours to fuse the fibers together, and then shaped to produce a rod having a diameter of 25 mm. The obtained rod was pushed into a tapered die heated to 210 ° C. having a diameter of 25 mm on the inlet side and a diameter of 1 mm on the outlet side, and a monofilament having a diameter of 0.82 mm was obtained by taking out the discharged fiber. The theoretical stretching ratio calculated from the diameter before and after stretching is 930 times.

    As a result of cross-sectional observation of the obtained monofilament, it was a sea-island type composite fiber containing countless island components made of thermoplastic polyacrylonitrile inside, and the island components were almost completely independent. Here, the average diameter of the island component was 87 nm, and the proportion of islands having a circularity of 0.9 or more was 97.5%.

Example 4
Sea-island type composite fibers were produced under the following conditions.
Island component: Novolac-type phenolic resin (Sumitomo Bakelite Co., Ltd. softening point 95 ° C)
Sea component: low density polyethylene (Mitsui Chemicals, Inc. Mirason (registered trademark) FL60 MFR = 70 melting point 102 ° C.)
Spinneret: 36 islands × 16 filaments Sea-island ratio: island / sea = 20/80 (weight ratio)
Total discharge amount: 12 g / min
Spinning temperature: 200 ° C
Winding speed: 200m / min
At the time of spinning, if the winding was attempted at a high speed, the fiber was broken. Therefore, it was necessary to perform spinning at a lower winding speed, but no yarn breakage was observed. The total fineness of the obtained undrawn yarn was 600 dTex. Here, when the cross-section observation of the undrawn yarn was performed, the size of the island component inside the fiber was considerably varied, and fibers with almost no island component were observed. Therefore, when the diameter of the island component was measured and averaged for 16 fibers, the average diameter of the island component was 4.2 μm. 240 fibers were aligned and filled in a heat-shrinkable tube, treated in a vacuum dryer at 170 ° C. for 3 hours to fuse the fibers together, and then shaped to produce a rod having a diameter of 10 mm. The obtained rod was pressed into a tapered die heated to 340 ° C. having a diameter of 10 mm on the inlet side and a diameter of 1 mm on the outlet side, and the discharged fiber was taken out to obtain a monofilament having a diameter of 0.92 mm. The theoretical stretching ratio calculated from the diameter before and after stretching is 121 times.

    As a result of cross-sectional observation of the obtained monofilament, it was a sea-island type composite fiber containing innumerable island components made of phenol resin inside, but the size of the island components varied considerably. Here, the average diameter of the island component was 463 nm, and the ratio of islands having a circularity of 0.9 or more was 97.8%.

Comparative Example 1
Sea-island type composite fibers were produced under the following conditions.
Island component: Mesophase pitch (Mitsubishi Gas Chemical AR resin, softening point 270 ° C, quinoline infusible 21%)
Sea component: polyethylene terephthalate ([η] = 0.63, melting point 260 ° C., melt viscosity at 310 ° C. 92 Pa · sec)
Spinneret: 36 islands × 16 filaments Sea-island ratio: island / sea = 20/80 (weight ratio)
Total discharge amount: 12 g / min
Spinning temperature: 310 ° C
Winding speed: 100m / min
At the time of spinning, the viscosity of the extruded fiber is very high, and if the winding is attempted at a high speed, the fiber breaks. Therefore, it is necessary to perform spinning at a very low winding speed. In addition, yarn breakage frequently occurred during the experiment due to the gas generated from the pitch. The obtained undrawn yarn had a total fineness of 1200 dTex and a single yarn fineness of 76 dTex. Further, when the cross section of the undrawn yarn was observed, an island component was observed uniformly inside the fiber and the diameter was 5.8 μm. 120 fibers were drawn and filled in a heat-shrinkable tube, treated in a vacuum dryer at 120 ° C. for 3 hours to fuse the fibers, and then shaped to produce a rod having a diameter of 10 mm. The obtained rod was pushed into a tapered die heated to 340 ° C. having a diameter of 10 mm on the inlet side and a diameter of 1 mm on the outlet side, and the discharged fiber was taken out to obtain a monofilament having a diameter of 0.86 mm. The theoretical stretching ratio calculated from the diameter before and after stretching is 164 times.

    As a result of cross-sectional observation of the obtained monofilament, it was a sea-island type composite fiber containing countless island components consisting of mesophase pitch inside, but the shape of the island component was indeterminate, and it was greatly out of the circle. It was. The average diameter of island components calculated by image processing was 529 nm, but the proportion of islands with a circularity of 0.9 or more was 3.3%.

  This sea-island type blended fiber was heated from room temperature to 250 ° C. at 1.5 ° C./min in an air atmosphere, and kept for 2 hours to be infusibilized. Thereafter, a carbon fiber was produced by heat treatment at 600 ° C. for 1 hour in a nitrogen atmosphere. When the produced carbon fiber was observed with an electron microscope, a carbon lump in which the carbon fibers were fused to each other was observed, and the carbon fibers could not be separated.

Comparative Example 2
First, an island component resin and a sea component resin were mixed under the following conditions to prepare a blend resin.
Island component: Thermoplastic polyacrylonitrile resin (Mitsui Chemicals, Barex (registered trademark) # 3000)
Sea component: Polylactide (Cargill Dow Polymer LLC, 6250D, melting point 170 ° C., melt viscosity at 220 ° C. 83 Pa · sec)
Sea-island ratio: island / sea = 20/80 (weight ratio)
Kneading temperature: 190 ° C
Screw rotation speed: 250rpm
Processing speed: 4kg / hour
Next, the obtained blended resin was melt-spun under the following conditions to produce a sea-island blend fiber.

Spinneret: 6 filaments Total discharge: 7.2 g / min
Spinning temperature: 195 ° C
Winding speed: 1200m / min
The obtained undrawn yarn had a total fineness of 60 dTex and a single yarn fineness of 10 dTex. This fiber was stretched 2.6 times using a roller-type stretching machine.

    When the cross section of the obtained fiber was observed, it was a sea-island type composite fiber containing countless island components made of thermoplastic polyacrylonitrile inside, and most of the island components were independent, but some island components were Fusing was observed. Here, the average diameter of the island component was 140 nm, and the ratio of islands having a circularity of 0.9 or more was 87.2%.

  This sea-island type blend fiber was fired in the same manner as in Example 1 to produce a carbon fiber. When the produced carbon fiber was observed with an electron microscope, a carbon fiber having a diameter of 80 to 150 nm was observed, but it was a spun yarn in which short fibers having a length of several μm were aggregated, and a continuous ultrafine carbon fiber was not obtained. It was.

It is a schematic diagram of the taper-shaped die | dye which can be used for the super drawing of a fiber.

Explanation of symbols

1: Die body 2: Fiber inlet 3: Fiber outlet

Claims (5)

  A sea-island composite fiber having an island component of a carbon raw material resin having melt shapeability and a sea component of a thermally decomposable resin, the ratio of the island component resin in the entire fiber is 10 to 50% by weight, and the diameter of the island component is A method for producing a composite fiber, which is produced by melt spinning so as to have a thickness of 1 to 10 μm, and the obtained fiber is super-stretched. The island component resin and the sea component resin are characterized in that the resin viscosity measured at a temperature higher by 50 ° C. than the higher melting point of each resin and at a shear rate of 1216 sec ^-1 satisfies the following formula. Manufacturing method of composite fiber.

η _s : Viscosity of sea component (thermally decomposable resin) η _i : Viscosity of island component (carbon raw material resin)   The method for producing a composite fiber according to claim 1 or 2, wherein the island component resin is an acrylonitrile-based resin capable of being melt-shaped.   The method for producing a composite fiber according to any one of claims 1 to 3, wherein the sea component resin is an aliphatic polyester.   The method for producing a composite fiber according to any one of claims 1 to 4, wherein the composite fiber is super-stretched by passing a heated tapered die after a plurality of composite fibers are bundled.

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