JPH0397917A - Modified cross-sectional carbon fiber and production thereof - Google Patents

Abstract

PURPOSE:To produce the subject fiber suitable as a reinforcing material for composite materials having excellent characteristics by spinning a spinning raw solution comprising an acrylonitrile based polymer with a spinneret having a specific shape, followed by coagulating, washing with water, drawing and baking. CONSTITUTION:A spinning raw solution comprising an acrylonitrile based polymer comprising >=95mol% of acrylonitrile and a solvent such as dimethyl sulfoxide is directly spun into a coagulating solution bath comprising the solvent and a coagulating agent such as water or methanol through a spinneret comprising point contact type extrusion holes. The coagulate fibers are washed with water and subsequently drawn in warm water, hot water, etc., and then baked to provide the objective fibers each having a non-circular cross section having a rotation symmetrical angle theta of 360 degree/n (n is 2-20) and having fine uneven structures thereon.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a modified cross-section carbon fiber and a method for producing the same, and more specifically to a modified cross-section carbon fiber suitable as a reinforcing material for composite materials having excellent properties and its production method. Regarding the manufacturing method.

(Prior art) Conventionally, carbon fibers have been widely produced industrially as fibers for reinforcing composite materials by taking advantage of their excellent mechanical properties, especially their specific strength and specific modulus. In material applications, particularly in sports and aerospace applications, there is an increasing demand for improved performance of carbon fibers.

In response to such demands for higher performance, numerous improvements have been made to date, and the use of composite materials using carbon fiber as a reinforcing material is expanding. However, although great progress has been made in the properties of carbon fiber itself, such as the strength and elastic modulus of resin-impregnated strands, the current situation is that the properties of composite materials that are actually used have not yet been fully satisfactory. be. That is, the problem is that the basic properties of the composite material, such as strength utilization rate, interglalabella shear strength (ILSS), compressive strength, and bending strength, do not improve.

There are many ways to solve these problems, including methods to improve the surface properties of carbon i-fibers by electrolytic surface treatment, methods to improve the properties of the applied matrix resin, and methods to improve the arrangement of carbon fibers that make up composite materials. Although methods for this have been disclosed, they do not innovatively improve the properties of the carbon fibers themselves that make up the composite material to improve the basic properties.

(Problem to be solved by the invention) The problem of the present invention is to have extremely excellent basic properties such as strength utilization rate, interlaminar shear strength (ILSS), compressive strength, and supporting strength in the composite material category, and to have an outstanding k reinforcement effect. The object of the present invention is to provide a carbon fiber shown in the figure as well as a method for producing the same.

(Means for Solving the Problems) The above problems of the present invention are as follows: (1) Strength of 300 kg/mm2 or more and 2OL/mm2 or more
In the case of carbon fibers having a slit modulus of mm2, the cross section of the fibers is non-circular with a rotational symmetry angle (°) of θ = 360''/n (n is an integer from 2 to 20), and '
A carbon fiber with an irregular cross section, which has no lamellar structure in the cross section of the fiber, has a substantially uniform crystal structure, and has a fine uneven structure on the surface of the fiber, and (2) ) A spinning dope consisting of an acrylonitrile polymer containing at least 95 mol % of acrylonitrile and a solvent for the polymer is passed through a spinneret having non-circular, slit-shaped, or point-contact discharge holes and the solvent and coagulant. The cross section of the fiber obtained by direct spinning into a coagulation bath, coagulation, water washing, and stretching has a rotational symmetry angle (°) of θ=360”/n (n is an integer from 2 to 20). This problem can be solved by a method for producing carbon fiber, which is characterized by firing a yarn having an irregular cross section.

The carbon fiber of the present invention ■ has a non-circular cross-sectional shape, ■ has no lamellar structure in the cross-section III and has a substantially uniform crystal structure, and ■ has a rough surface. The three most important features are the presence of a microscopic uneven structure.

Many techniques have been disclosed so far for carbon III having a non-circular or irregular cross-section, particularly for pitch-based carbon fibers. For these pitch-based carbon fibers, great efforts have been made to the pitch as a raw material in order to maintain basic properties such as elastic modulus and strength, and it has become common to use mesophase pitch. There is. When these mesophase pitches are used as raw materials, shear stress during spinning during production of raw fibers (bricasa) creates a non-uniform crystal structure inside the raw fibers, which causes the internal structure of the carbon fibers after firing. However, it remains as a non-uniform crystal structure observed as a lamellar structure, resulting in a decrease in strength and elastic modulus. In order to improve the defects caused by the basic characteristics of raw materials, the creation of irregular cross-sections has been devised. For example, JP-A-61-6313, JP-A-62-
117821, JP-A-62-231024, JP-A-32-131034, and the like. However, in these disclosed techniques, the above-mentioned non-uniform crystal structure still exists and no improvement in compressive strength can be expected. A leaf-like lamellar structure is observed in these irregular cross-section carbon fibers along the cross-sectional shape of the fibers.

The presence or absence of this leaf-like lamellar structure can be determined, for example, by
(As described in Publication i313 and Japanese Patent Application Laid-open No. 117821/1982, when the cross section (fractured surface) of carbon fiber is observed using a scanning electron microscope (SEM),
This can be easily observed as a fractured Mi weave extending radially outward from the center of the fracture surface. Figure 1 shows a schematic diagram of the leaf-like lamellar structure that appears on the fracture surface.

The irregular cross-section carbon fiber of the present invention does not have a nonuniform crystal structure corresponding to the leaf-like lamella structure described above, and exhibits a substantially uniform internal structure.

For such irregular cross-section carbon fibers with uniform internal structure, for example, 20tlII International
SAMPE Technical Conference
(1988) Lecture Proceedings P414-422 describes that polyacrylonitrile (PAN) can be produced by firing raw yarn obtained by melt spinning, and a cross-sectional photograph is published. However, although there is a description of the characteristics of carbon fiber with a circular cross section in the proceedings,
The specific properties of the irregular cross-section carbon fibers were not described, and it was not clear at all what properties and features they would exhibit.

The irregular cross-section carbon fiber of the present invention simultaneously satisfies the properties (1), (2), and (2) above, and the strength, which is an important basic property, is 300 kg/mm2 or more when measured in the form of a strand, which will be described later. , preferably 320 kg/mm2 or more. Similarly, the elastic modulus is 204/mrn2 or more, preferably 22t/B
"The above is the above. Unless both the strength and elastic modulus reach this value, the advantages as a reinforcing material for composite materials cannot be exhibited. In particular, since the irregular cross-section carbon 111i1I of the present invention has a non-circular cross section, the composite material A The basic properties such as interlayer shear strength (ILSS), compressive strength, and bending strength, which are important properties of the material, are extremely excellent.

The cross-sectional shape of the carbon fibers of the present invention is different from the bean-shaped or cocoon-shaped carbon fibers produced by conventional carbon fibers produced by firing raw fibers spun from circular discharge holes.

θ=360''/n (n is 2 to 20
The cross-sectional shape of the carbon wc fiber, which is a non-circular shape with a rotational index angle θ (°) of up to an integer up to This includes multi-lobed shapes such as , and alphabetic shapes such as II. For reference, these polygonal (multilobal) shapes and 1
The relationship between 1 and θ is shown below.

n θ (°) Triangle 3l2〇Square 490 FIG. 2 shows an example of the shape of the discharge hole of the spinneret and the cross-sectional shape of the carbon fiber obtained. Among these, polygonal and multilobal shapes are preferred from the viewpoint of ease of production. In addition, it is important to set the ratio of leaves too high or too long, especially in the case of multi-lobed products, from the viewpoint of the characteristics obtained and the ease of passing through the manufacturing process. It is preferable that the ratio (R/r) of the diameters of the circumscribed circle and the inscribed circle (r and R, respectively) of the cross section is within an appropriate range. FIG. 3 shows, as an example, the relationship between R and r in the Y cross section. R/r is preferably 1.1-7.0, more preferably 1.2-G. 0, more preferably 1.3-5
．． It is 0.

The cross section of the irregular cross-section carbon fiber string of the present invention has a substantially uniform crystal structure, and there is no leaf-like lamellar structure, which is extremely advantageous in obtaining high-performance carbon fibers. . Such a homogeneous structure without a lamellar structure can be easily obtained by firing a raw yarn made of a polyacrylonitrile polymer.

There are minute irregularities on the side surfaces of the irregular cross-section carbon fiber of the present invention, but this feature is unique to the manufacturing method of the present invention! iIl
It is closely related to the formation mechanism of i-fibers, and this point, along with the absence of a leaf-like lamellar structure in the cross section, is clearly different from pitch-based carbon fibers with irregular cross sections. These fine irregularities on the surface function extremely effectively in improving the adhesion with the matrix resin when a composite material is formed.

Acrylic i! used in the present invention! The production of II contains acrylonitrile (hereinafter referred to as AN) as the main component,
95 mol% or more, preferably 98 mol% or more of AN, and preferably 5 mol% or less, particularly preferably 2 mol%
A vinyl group-containing compound (hereinafter referred to as a vinyl monomer) that is copolymerizable with the following AN and promotes the ml flame reaction.
A copolymer with is used.

Examples of vinyl monomers that promote flame resistance include acrylic acid, methacrylic acid, itaconic acid, and their alkali metal salts, ammonium salts, and α(l).
-hydroxylethyl) acrylonitrile, acrylic acid hydroxyl ester, and the like. In addition to these vinyl monomers that have the ability to promote inflammation, AN
Spinnability of polymers or! ! In order to improve thread properties etc., a third component such as lower alkyl esters of acrylic acid or methacrylic acid, allylsulfonic acid, methallylsulfonic acid, styrenesulfonic acid and their alkali metal salts, vinyl acetate or vinyl chloride, etc. The total amount of copolymerized components is 5
Copolymerization may be carried out within a range of mol % or less, preferably 2 mol % or less.

Such AN-based copolymers can be polymerized using emulsion suspension, bulk, solution, and other polymerization methods.

To produce acrylic fibers from these polymers, dimethylformamide, dimethyl sulfoxide, nitric acid,
A volima solution consisting of a solvent such as a rhodan soda aqueous solution and a zinc chloride aqueous solution is used as a spinning stock solution.

The spinning method in the method for producing irregular cross-section carbon fibers of the present invention involves spinning a spinning nozzle with a non-circular, slit-shaped, or point-contact discharge hole imitating the cross-sectional shape of the raw fiber into a coagulation bath consisting of the solvent and a coagulant. This is a wet spinning method that involves spinning directly into the fiber. The coagulated yarn thus obtained has a thin surface skin layer, and a fibril structure can be developed on the surface of the yarn in the subsequent drawing step. The general composition of coagulation consists of the above-mentioned PAN solvent and a coagulant. Examples of the coagulant include water, methanol, ethanol, acetone, etc., but water is suitable from the viewpoint of safety and recovery. The obtained coagulated fibers are washed with water and then stretched in warm water or hot water, and then a process oil is applied at a rate of 0.2 to 1.0% per dry fiber weight.
Add 5% by weight. As a component of the oil agent, it is preferable to add a silicone-based compound or a modified silicone-based compound, which is particularly effective in preventing fusion of single fibers during firing.

After applying the process oil, dry densification treatment is performed,
IIav! Transformation! Obtain l fiber. Next, if necessary, secondary stretching is performed, for example, in steam.

By the way, the single yarn weave of the raw yarn is the obtained irregular cross-section carbon! il
li! This is an extremely important factor in defining the characteristics of I. In the present invention, the denier is preferably 0.1 to 2.5 denier, more preferably 0.2 to 2.0 denier, and still more preferably 0.3 to 1.5 denier. If it is smaller than 0.1 denier, single filament breakage will easily occur, while if it exceeds 2.5 denier, uniform firing of the inner and outer layers of the single filament will not be possible, making it difficult to obtain carbon fiber with excellent properties. .

The acrylic fibers (hereinafter referred to as breaker) produced in this manner are fired and converted into carbon fibers. Although the conditions for firing the breaker, ie, oxidation (flame resistance) and carbonization, are not particularly limited, it is preferable to set conditions that make it difficult for structural defects such as voids to occur inside the fibers. For example, the conditions for carbonization in an inert atmosphere such as nitrogen are 300-700℃ and 1
The temperature increase rate in the temperature range of 000 to 1200℃ is 1
000°C/min or less, preferably 500°C/min or less. Furthermore, for example, 1400℃~
It is also possible to obtain a graphitized yarn by firing in an inert atmosphere at 3000°C.

(Example) The present invention will be explained in more detail with reference to Examples below.

Note that the physical property values used in the text and examples were measured by the following method.

(A) Resin-impregnated strand strength measurement method JISR-76
It was measured according to the resin-impregnated strand test method specified in 01.

・“Bakelite” ERL-4221 100
3 parts 3-fluoroboronoethylamine ([3F3MEA) 3 parts acetone 4 parts Curing conditions: l
30°C x 30 minutes (B) Characteristic values for composite material a. Tensile strength, bending strength, glabellar shear strength (l., SS) The carbon fiber wound around the metal frame is 60% carbon fiber volume content (Vf)
After pouring the resin into a mold, it is heated and degassed under vacuum.

After defoaming, heat the C resin while applying pressure with a press to harden it and create a test piece. Measured using an Instron testing machine and converted to Vf = 60%.

Main resin: ELM434 [Sumitomo Chemical Co., Ltd.] 35 parts Ep
828 [Petrochemicals Co., Ltd.] 35 parts Ebiclone +
52 [Dainippon Ink Co., Ltd.] 30 parts 4,42-diaminodiphenylsulfone [Sumitomo Chemical (
Ltd.] 32 parts 3 boron fluoride monoethylamine 0.5 parts (solvent: methyl ethyl ketone, resin concentration = 55%) Main molding conditions Defoaming: Molding at 70°C for 2 hours under vacuum (10 mmllg or less): Press pressure = 50 kg / cm”, 170
℃ × 1 hour post cure: After taking out the test piece from the mold, 170℃
℃ × 2 hours Molding: Tensile test: Width (3 mm, thickness I.Gmm dB strength, ILSS; Width (3 mm, thickness 2.5 mm * Measurement tensile strength: The length of the test piece was 150 mm, and 45 questions were Glue the aluminum tabs.

A radius of 75 mm is machined from both sides in the thickness direction of the center of the test piece, and the test piece is used for measurement.

The cross-sectional area is determined by measuring the thickness and width of the thinnest part of the rounded part.

West strength: The length of the test piece is 150 nm, 3 points I
Measure using a test jig.

ILSS: The length of the test piece is 18n+m, and the support span is measured as four times the thickness of the test piece using a three-point auxiliary test jig.

b. Compressive Strength Carbon fibers were arranged in one direction on a resin film made by coating silicone resin-coated paper with #3620 resin manufactured by Toray Industries, Inc., and the resin film was layered on top of it again, and the resin was applied with a pressure roll. It is impregnated into carbon fiber to create a Bripre resin. These sheets are stacked with their fiber axes aligned and treated using an autoclave at a temperature of 180° C. and a pressure of 6 kg/crn2 for 2 hours to harden the resin, thereby creating a flat plate with a thickness of approximately 1 mm. This flat plate was cut using a diamond cutter to a length of 80 m in the fiber axis direction.
A test piece having a width of 12n+m in the direction perpendicular to the fiber axis is prepared. Leave 5 mm in the center of the test piece, and glue composite tabs made of carbon fiber and epoxy resin with a thickness of about 1 inch to both ends to prepare a test piece for compressive strength measurement. (C) Observation of the fracture surface using a scanning electron microscope (SEM) The carbon fiber single yarn to be measured was measured using a tensile tester under the following conditions to prevent the fracture surface from being complicatedly disturbed by the impact of the fracture. break in water.

Chuck interval: 5 cm Tensile speed: 0.5 mm/min [1% strain/min co-sample length: 15 cm Gold was deposited on the obtained sample, and the accelerating voltage was 15 ~
25kv (25kv), magnification 5000-S-1500
Observe or photograph with SEM at 0x (10000x). The values in parentheses are the measurement conditions in this example.

Example-1 AN copolymer dimethyl sulfoxide (hereinafter referred to as AN) with an intrinsic viscosity [η of 1.80, consisting of 99.5 mol% acrylonitrile (hereinafter referred to as AN) and 0.5 mol% itaconic acid
Ammonia is blown into the DMSO solution and substituted with an ammonium cum group, which is the carboxyl end group of the copolymer, to modify the polymer into a DMSO solution, and the resulting modified polymer has a concentration of 20% by weight. And so.

This spinning dope was directly discharged at 50°C into a 55% DMSO aqueous solution at 50°C through a spinneret with 1,500 holes (Y hole, 10 holes, and 11 holes with a slit width of 0.03 mm) to solidify. I made it into a thread. For comparison, the diameter is 0.06
A coagulated thread was similarly obtained using a conventional die having a circular discharge hole of mm.

After washing the coagulated yarn with water, it was drawn in three stages in warm water.
A bath-drawn yarn was obtained. The stretching ratio was 3.0 times as a whole, and the maximum temperature of the stretching pond was 98°C. Next, an oil agent containing a modified silicone compound as one component was applied to the bath-rolled medium yarn, and then it was dried and densified using a heating roll at 130°C. Subsequently, it was drawn 3.5 times in pressurized steam to obtain an acrylic fiber yarn having a single yarn weave of 1.0 denier and a total fineness of 1500 denier.

This acrylic fiber thread was subjected to flame reduction treatment while being stretched 1.05 times in air at 240 to 260°C, and then stretched to 300°C in a nitrogen atmosphere with a maximum temperature of 1400°C.
The heating rate in the temperature range of ~700℃ is 250℃/min.
The temperature increase rate in the temperature range of IO00 to I200℃ is set to 40
Processing was carried out in a carbonization furnace set at 0°C/min to convert it into dark blue. Furthermore, the carbon fiber obtained here is 1600~3
Graphite yarns having various elastic moduli were obtained by graphitization in a Tammann furnace at 000°C.

In order to improve compatibility with the resin, appropriate anodization treatment was performed in a sulfuric acid or nitric acid aqueous solution.

When the monofilament of the irregular cross-section carbon fiber obtained here was broken in water and the broken surface was observed with an SEM, no lamellar structure on the leaf was observed in any of them. Furthermore, when the side surfaces of the fibers were similarly observed using SEM, fine uneven structures were observed in all cases.

Table 1 shows the strand properties and composite properties. Furthermore, FIG. 4 shows the relationship between the elastic modulus and compressive strength of circular cross-section carbon fibers and Y-section carbon W4 fibers. From FIG. 4, it can be seen that the navy blue irregular cross-section collapsible fiber of the present invention exhibits superior compressive strength than the circular cross-section carbon m-fiber.

(Effects of the Invention) The irregular cross-section carbon fiber of the present invention has: ■ The cross-sectional shape of the wA fiber is non-circular, ■ No lamellar structure exists in the cross-section of the fiber, and it has a substantially uniform crystal structure. ■There is a microscopic uneven structure on the surface of the fiber, and ■The strength is 300 kg/
mm” or more, and has an elastic modulus of 201/mm2 or more.Irregular cross-section carbon I6l having these characteristics
i-fibers can be produced by the method of the present invention, and the important properties are open shear strength (ILSS), compressive strength, t
A composite material with extremely excellent basic properties such as U+ strength can be obtained.

[Brief explanation of drawings]

FIG. 1 shows an example of a schematic diagram of a leaf-like lamellar structure that appears on the fracture surface of a pitch-based carbon fiber. FIG. 2 shows an example of the shape of the discharge hole of the spinneret and the cross-sectional shape of the obtained carbon FIAllI. Figure 3 shows the relationship between R and r in Y cross-section carbon fiber. FIG. 4 shows the relationship between the elastic modulus and compressive strength of circular cross-section carbon fibers and Y-section carbon fibers in Examples. Figure 1 Figure 3 Figure 2

Claims (2)

[Claims] (1) Strength of 300kg/mm^2 or more and 20t
In a carbon fiber having an elastic modulus of /mm^2, the cross section of the fiber is non-circular with a rotational symmetry angle (°) of θ = 360°/n (n is an integer from 2 to 20), and the fiber An irregular cross-section carbon fiber characterized by having no lamellar structure in its cross section, having a substantially uniform crystal structure, and further having a fine uneven structure on the surface of the fiber. (2) A spinning dope consisting of an acrylonitrile polymer containing at least 95 mol % of acrylonitrile and a solvent for the polymer is passed through a spinneret having non-circular, slit-shaped, or point-contact discharge holes. The cross section of the fiber obtained by direct spinning into a coagulation bath consisting of a coagulant, coagulation, water washing, and stretching has a rotational symmetry angle (°) of θ = 360°/n (n is an integer from 2 to 20). 1. A method for producing carbon fiber, comprising firing a yarn having an irregular cross section.