JP2012211410A - Quality control method of carbon fiber precursor acrylic fiber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a more simple method for distinguishing a carbon fiber precursor fiber that contains an iron element in an amount that causes a decline in strength.SOLUTION: The quality control method of a carbon fiber precursor acrylic fiber includes at least the following steps (1) to (5): (1) a step of burning a carbon fiber precursor acrylic fiber that contains less than 0.5 ppm of iron and a carbon fiber precursor acrylic fiber that contains 0.5 ppm or more of iron under the same burning condition to obtain carbon fibers; (2) a step of obtaining strand strength of respective fibers obtained in the step (1) to determine the iron content with which the strength lowering rate of the strand strength of a carbon fiber obtained by burning the carbon fiber precursor acrylic fiber that contains 0.5 ppm or more of iron to the strand strength of a carbon fiber obtained by burning the carbon fiber precursor acrylic fiber that contains less than 0.5 ppm of iron becomes 0.5% or more and 20% or less; (3) a step of applying a heat treatment to the carbon fiber precursor acrylic fiber that contains less than 0.5 ppm of iron and the carbon fiber precursor acrylic fiber that contains iron such that the strength lowering rate becomes 0.5% or more and 20% or less under the same conditions, at a temperature of 180 to 300°C and for 1 to 20 minutes; (4) a step of obtaining a color difference of the heat treated fibers obtained in the step (3) by a color difference meter; and (5) a step of detecting a process failure based on the color difference obtained in the step (4).

Description

  The present invention relates to a quality control method for a carbon fiber precursor acrylic fiber.

  It is known that carbon fibers have a high specific strength and specific elastic modulus compared to other fibers. For this reason, in addition to conventional sports and aerospace applications, composite fibers are being widely deployed in general industrial applications such as automobiles, civil engineering, architecture, pressure vessels, and windmill blades. Furthermore, there is a high demand for higher strength and higher elastic modulus in sports and aerospace applications that have been used in the past.

  Among the carbon fibers, polyacrylonitrile-based carbon fibers are the most widely used. The polyacrylonitrile-based carbon fiber is converted into a flame-resistant fiber by, for example, using an acrylic fiber with an oil agent as a carbon fiber precursor, and heat-treating the carbon fiber precursor in an oxygen-containing atmosphere at 200 to 400 ° C. Subsequently, it is obtained by carbonization under an inert atmosphere of 1000 ° C. or higher. The carbon fiber obtained by this method is widely used industrially as a reinforcing fiber for composite materials because of its excellent mechanical properties.

  An iron element is cited as a factor that affects the strength and elastic modulus of carbon fibers (Patent Document 1). Therefore, there is a need for a quality control method for maintaining the amount of iron element that does not affect the strength in the carbon fiber precursor.

JP-A-5-156523

  In order to measure the amount of iron element in acrylic fiber, the amount of iron element can be quantified by atomic absorption or ICP measurement after ashing and dissolution, but it takes time and labor to quantitate. It takes time, which is not preferable because a product loss increases until a situation in which the amount of iron element exceeds a specified value is detected, resulting in an increase in product cost. It takes time and effort, which is undesirable because it rebounds on the product cost.

  Therefore, an object of the present invention is to provide a method for discriminating carbon fiber precursor fibers containing an iron element in an amount that causes a decrease in strength more easily.

  As a result of intensive studies, the present inventors have found that when an amount of iron element that causes a decrease in strength is mixed in the carbon fiber precursor acrylic fiber, the corresponding fiber is heat-treated and the same treatment is performed on the normal carbon. The present inventors have found that there is a difference in color when compared with the fiber precursor acrylic fiber, leading to the present invention.

That is, the following steps (1) to (5) are performed to control the quality of the carbon fiber precursor acrylic fiber.
(1) A step of firing carbon fiber precursor acrylic fiber containing less than 0.5 ppm of iron and a carbon fiber precursor acrylic fiber containing more than 0.5 ppm of iron under the same firing conditions to obtain carbon fiber,
(2) The strand strength of each carbon fiber obtained in (1) above is determined, and the strand strength of the carbon fiber obtained by firing the carbon fiber precursor acrylic fiber containing less than 0.5 ppm of the iron content. A step of determining the iron content at which the strength reduction rate of the strand strength of the carbon fiber obtained by firing the carbon fiber precursor acrylic fiber containing 0.5 ppm or more of the iron content is 0.5% or more and 20% or less,
(3) A carbon fiber precursor acrylic fiber containing less than 0.5 ppm of iron and a carbon fiber precursor acrylic fiber containing iron that has a strength reduction rate of 0.5% or more and 20% or less. A step of heat-treating at the same conditions for 1 minute to 20 minutes at a time;
(4) A step of obtaining a color difference of the heat-treated fiber obtained in (3) with a color difference meter,
(5) A step of detecting a process abnormality based on the color difference obtained in (4).

  According to the present invention, it is possible to more easily identify iron element contamination in a carbon fiber precursor acrylic fiber that causes a decrease in strength of the carbon fiber during firing.

(Quality control method by color difference)
The following (1) to (5) are performed to control the quality of the carbon fiber precursor acrylic fiber.
(1) A step of firing carbon fiber precursor acrylic fiber containing less than 0.5 ppm of iron and a carbon fiber precursor acrylic fiber containing more than 0.5 ppm of iron under the same firing conditions to obtain carbon fiber,
(2) The strand strength of each carbon fiber obtained in (1) above is determined, and the strand strength of the carbon fiber obtained by firing the carbon fiber precursor acrylic fiber containing less than 0.5 ppm of the iron content. A step of determining the iron content at which the strength reduction rate of the strand strength of the carbon fiber obtained by firing the carbon fiber precursor acrylic fiber containing 0.5 ppm or more of the iron content is 0.5% or more and 20% or less,
(3) A carbon fiber precursor acrylic fiber containing less than 0.5 ppm of iron and a carbon fiber precursor acrylic fiber containing iron that has a strength reduction rate of 0.5% or more and 20% or less. A step of heat-treating at the same conditions for 1 minute to 20 minutes at a time;
(4) A step of obtaining a color difference of the heat-treated fiber obtained in (3) with a color difference meter,
(5) A step of detecting a process abnormality based on the color difference obtained in (4).

  The above method is necessary because the color difference changes depending on the heat treatment conditions during management, and the strength reduction rate required according to the strength level of the carbon fiber also changes.

  The strength reduction rate is preferably 0.5% or more and 20% or less. More preferably, they are 1% or more and 15% or less, More preferably, they are 1% or more and 10% or less. Within this range, it is possible to determine an undesired drop in strength without affecting the error range.

(Heat treatment conditions)
Although the temperature of heat processing is not specifically limited, The range of 180 to 300 degreeC is preferable, and between 190 to 280 degreeC is more preferable. 200 degreeC-240 degreeC is further more preferable. Within this range, the color difference is easy to distinguish. The atmosphere for the heat treatment is not particularly limited, but is preferably in the air.

  Although the time of heat processing is not specifically limited, 1 minute-20 minutes are preferable, 3 minutes-15 minutes are more preferable, and 5 minutes-12 minutes are further more preferable. If it is within the range, it is easy to distinguish the color difference.

  The conditions for heat-treating each fiber must be aligned. It is preferable that the fibers to be compared are aligned and heat-treated at the same time. It is preferable to align the fineness of the target fiber and the fiber to be inspected, and the difference between the fineness of the target fiber is preferably 50% or less, more preferably 20% or less, More preferably, it is 10% or less. It is preferable to arrange the basis weight, and the difference is preferably 50% or less, more preferably 20% or less, and even more preferably 10% or less, with respect to the fineness of the target fiber. .

(Carbon fiber precursor acrylic fiber)
The carbon fiber precursor acrylic fiber is a fiber obtained by spinning an acrylonitrile polymer.

  The acrylonitrile-based polymer used in the present invention is a polymer obtained by polymerizing acrylonitrile as a main monomer. The acrylonitrile-based polymer is not limited to a homopolymer obtained only from acrylonitrile, but may be an acrylonitrile-based polymer using other monomers in addition to the main component acrylonitrile.

  The composition ratio of acrylonitrile in the acrylonitrile-based polymer can be determined in consideration of the quality required for the obtained carbon fiber, and is preferably 96 to 98.5% by mass, for example. If the composition ratio of acrylonitrile is 96% by mass or more, the excellent quality and performance of the carbon fiber can be maintained without causing the fusion of the fibers in the firing step for converting the precursor fiber into the carbon fiber. In addition, the heat resistance of the acrylonitrile polymer does not decrease, and drying can be suppressed when the precursor fiber is spun. Furthermore, adhesion between single fibers can be avoided in processing such as stretching with a heating roller or pressurized steam. If the composition ratio of acrylonitrile is 98.5% by mass or less, the solubility in a solvent does not decrease, precipitation / coagulation of acrylonitrile polymer can be prevented, and the stability of the spinning dope can be maintained. Can be manufactured stably.

  A monomer other than acrylonitrile can be copolymerized with the acrylonitrile-based polymer. As a monomer other than acrylonitrile, a vinyl monomer that can be copolymerized with acrylonitrile can be selected as appropriate, and a vinyl monomer that improves the hydrophilicity of the acrylonitrile polymer, and a flame resistance promoting effect. The vinyl-type monomer which has is preferable.

Examples of the monomer that improves the hydrophilicity of the acrylonitrile-based polymer include vinyl compounds having a hydrophilic functional group such as a carboxyl group, a sulfo group, an amino group, and an amide group. Examples of the monomer having a carboxyl group include acrylic acid, methacrylic acid, itaconic acid, crotonic acid, citraconic acid, ethacrylic acid, maleic acid, mesaconic acid and the like, among which acrylic acid, methacrylic acid, and itaconic acid are preferable. Examples of the monomer having a sulfo group include allyl sulfonic acid, methallyl sulfonic acid, styrene sulfonic acid, 2-acrylamido-2-methylpropane sulfonic acid, vinyl sulfonic acid, and sulfopropyl methacrylate. Acid, methallylsulfonic acid, styrenesulfonic acid, and 2-acrylamido-2-methylpropanesulfonic acid are preferred. Examples of the monomer having an amino group include dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate, dimethylaminoethyl acrylate, diethylaminoethyl acrylate, tertiary butylaminoethyl methacrylate, allylamine, o-aminostyrene, and p-aminostyrene. Of these, dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate, dimethylaminoethyl acrylate, and diethylaminoethyl acrylate are preferable. As the monomer having an amide group, acrylamide, methacrylamide, dimethylacrylamide, and crotonamide are preferable. By copolymerizing such monomers, the hydrophilicity of the acrylonitrile polymer is improved. When the hydrophilicity is improved, the density of the obtained precursor fiber is improved, and generation of microvoids in the surface layer portion can be suppressed. The above-mentioned monomers can be used alone or in combination of two or more.
As for the composition ratio of the monomer which improves the hydrophilicity of such an acrylonitrile-type polymer, 0.5-3.5 mass% is preferable in an acrylonitrile-type polymer.

Examples of the monomer having an effect of promoting flame resistance include acrylic acid, methacrylic acid, itaconic acid, crotonic acid, citraconic acid, ethacrylic acid, maleic acid, mesaconic acid, or alkali metal salts or ammonium salts thereof, acrylamide, methacrylamide, etc. Is mentioned. Among them, a monomer having a carboxyl group is preferable from the viewpoint of obtaining a higher flame resistance-promoting effect with a small amount of blending, and a carboxyl group-containing vinyl monomer such as acrylic acid, methacrylic acid, and itaconic acid is more preferable. . By copolymerizing such a monomer, the time required for the flameproofing step described later can be shortened, and the production cost can be reduced. The above-mentioned monomers can be used alone or in combination of two or more.
The composition ratio of the monomer having the effect of promoting flame resistance is preferably 0.5 to 2.0% by mass in the acrylonitrile-based polymer.

Precursor fibers are those to which an oil composition is attached.
The oil agent composition can be determined in consideration of the function required for the precursor fiber, and for example, a silicone oil agent composition is preferable. Examples of the silicone-based oil composition include silicone oils such as amino-modified silicone and epoxy-modified silicone. Of these, amino-modified silicone is preferable. Examples of the amino-modified silicone include side-chain primary amino-modified silicone, side-chain primary and secondary amino-modified silicone, and both-end amino-modified silicone. By using such a silicone-based oil composition, it is possible to obtain carbon fibers that can be produced with high production efficiency and have excellent mechanical properties by increasing the fiber converging property in the spinning process.

  The adhesion amount of the oil composition in the precursor fiber is preferably 0.1 to 2.0% by mass, and more preferably 0.5 to 1.5% by mass with respect to the dry mass of the precursor fiber. preferable. When the adhesion amount of the oil composition is less than 0.1% by mass, it may be difficult to sufficiently develop the function of the oil composition. When the adhesion amount of the oil agent composition exceeds 2.0% by mass, the excessively adhered oil agent composition may be polymerized in the firing step to cause adhesion between single fibers.

(Addition of iron element)
The iron element contained or attached to the precursor fiber is an iron element contained or attached in any form such as iron ions and iron compounds. The method of adding the iron element is not particularly limited, but a method of adding a metal salt such as iron (III) nitrate in the stock solution step in which the polymer is dissolved in a solvent, or a metal salt such as iron (III) nitrate is added to the oil agent. By adhering, the iron element can be held in the carbon fiber precursor acrylic fiber.

(Production method)
The method for producing a precursor fiber of the present invention includes a stock solution step, a spinning step of spinning an acrylonitrile-based polymer solution, and a step of impregnating the obtained fiber with the oil dispersion.

[Undiluted solution process]
The acrylonitrile polymer solution is a spinning dope obtained by dissolving an acrylonitrile polymer solution in a solvent.
The solvent can be determined in consideration of the type of acrylonitrile polymer, and examples thereof include organic solvents such as dimethylacetamide, dimethylsulfoxide, dimethylformamide, and aqueous solutions of inorganic compounds such as zinc chloride and sodium thiocyanate. Acetamide, dimethyl sulfoxide, and dimethylformamide are preferable in that a dense precursor fiber can be obtained.

  The concentration of the acrylonitrile polymer in the acrylonitrile polymer solution is not particularly limited, but is preferably 17 to 25% by mass, and more preferably 19 to 25% by mass. If the content is 17% by mass or more, a dense coagulated yarn can be obtained, and if it is 25% by mass or less, an appropriate viscosity and fluidity can be obtained as a spinning dope.

[Spinning process]
The spinning step is a step of spinning an acrylonitrile polymer solution to obtain fibers (coagulated yarn). As the spinning method, for example, a wet spinning method in which spinning is directly performed in a coagulation bath, a dry spinning method in which coagulation is performed in air, and a dry wet spinning method in which spinning is performed once in air and then coagulated in a coagulation bath. For example, a known spinning method can be used. Among these, from the viewpoint of further improving the strength and elastic modulus of the carbon fiber, the wet spinning method or the dry wet spinning method is preferable.

  Examples of the spinning shaping by the wet spinning method or the dry-wet spinning method include a method in which the acrylonitrile-based polymer solution is spun into a coagulation bath from a nozzle having a substantially circular cross section.

  As the coagulation bath, it is preferable to use an aqueous solution containing a solvent used for an acrylonitrile-based polymer solution. Such a coagulation bath is preferred from the viewpoint of easy solvent recovery.

  When using the aqueous solution containing a solvent as a coagulation bath, the solvent concentration in the aqueous solution is preferably 50 to 85% by mass. If it is in the said range, a precursor fiber will be made into the precise | minute structure without a void generation | occurrence | production, and a carbon fiber with high strength and a high elastic modulus can be obtained. In addition, stretchability can be secured and productivity is excellent.

  Although the temperature of a coagulation bath is not specifically limited, 10-60 degreeC is preferable. If it is in the said range, a precursor fiber will be made into the precise | minute structure without a void generation | occurrence | production, and a carbon fiber with high strength and a high elastic modulus can be obtained. In addition, stretchability can be secured and productivity is excellent.

  In the spinning process, the coagulated yarn can be drawn in a coagulation bath or a drawing bath. Alternatively, the coagulated yarn can be drawn in the air and then drawn again in the bath. Alternatively, the coagulated yarn can be made into a water-swollen state by washing with water before or after stretching or during stretching.

  Examples of the stretching bath include water and an aqueous solution containing a solvent used for an acrylonitrile-based polymer solution.

  Drawing is performed by putting the coagulated yarn in a coagulation bath of 50 to 98 ° C. or a drawing bath and applying tension to the coagulated yarn. For example, the stretching may be performed at a desired ratio once, or may be performed at a desired ratio by stretching in two or more stages. For example, it is preferable that stretching in the air and stretching in the stretching bath are combined and stretched 5 to 15 times in total. By stretching in this way, the carbon fiber can be increased in strength and elastic modulus.

[Oil agent adhesion process]
The oil agent attaching step is a step in which the fiber obtained in the spinning step is impregnated with the oil agent dispersion and the oil agent is attached. Although the kind of oil agent is not specifically limited, Amino silicone type oil agent is used suitably.
The fiber to which the oil agent is attached is a coagulated yarn, and is preferably a coagulated yarn in a water-swollen state obtained after stretching in the bath, or after stretching in the bath or washing.

As a method for impregnating the coagulated yarn with the oil dispersion, known methods such as a roller method, a guide method, a spray method, a dip method, and the like can be used. The roller method is a method in which a roller is placed so that its axial direction is horizontal, the lower part of the roller is immersed in an oil dispersion, and the coagulated yarn is brought into contact with the upper part of the roller. In the guide method, a fixed amount of oil dispersion is discharged from a guide by a pump, and the coagulated yarn is brought into contact with the surface of the guide. In the spray method, a certain amount of oil dispersion is sprayed from a nozzle onto a coagulated yarn. In the dip method, the coagulated yarn is immersed in the oil dispersion and then squeezed with a roller or the like to remove excess oil dispersion.
From the viewpoint of uniform adhesion, a dip method is preferred in which the coagulated yarn is sufficiently impregnated with the oil dispersion and the excess oil dispersion is removed.

In the oil agent attaching step, the impregnation of the oil agent dispersion may be performed once by the above-described method, or may be a multistage process in which the above-described method is repeated twice or more. From the viewpoint of more uniformly attaching the oil agent to the coagulated yarn, multistage treatment is preferable.
Thus, a precursor fiber in which an oil agent is attached to the coagulated yarn can be produced.

[Drying process]
The precursor fiber of the present invention may be subjected to the production of carbon fiber as it is obtained in the oil agent attaching step, or may be provided with a drying step after the oil attaching step to dry and densify the precursor fiber. .

  A drying process can dry precursor fiber by a conventionally well-known method, for example, drying with a heating roller is mentioned as a preferable drying method. Note that the number of heating rollers may be one or two or more.

  The drying temperature in the drying step is preferably a temperature exceeding the glass transition temperature of the precursor fiber. By treating at such a drying temperature, drying and densification of the precursor fiber can be achieved. Although drying temperature changes with fluctuation | variation of the moisture content of a precursor fiber, it is preferable to determine in the range of 100-200 degreeC, for example.

  In the drying step, the precursor fiber can be dried and then subjected to pressurized steam stretching. By performing the steam stretching under pressure, the denseness and orientation degree of the precursor fiber can be further increased, and the carbon fiber can be further increased in strength and elastic modulus. Pressurized steam stretching is a method of stretching in a pressurized steam atmosphere. According to pressurized steam drawing, drawing at a high magnification is possible, stable spinning can be performed at a higher speed, and at the same time, it contributes to improving the denseness and orientation degree of the obtained fiber.

  The pressurized steam stretching includes, for example, a method in which the precursor fiber is preheated with a heating roller and then tension is applied to the precursor fiber in the presence of pressurized steam. In such pressurized steam stretching, it is preferable to preheat the precursor fiber by setting the temperature of the heating roller immediately before the pressurized steam stretching apparatus to 120 to 190 ° C. When the temperature of the heating roller is less than 120 ° C., the temperature of the precursor fiber does not rise sufficiently and the drawability is lowered.

  Moreover, it is preferable to control the fluctuation rate of the water vapor pressure in the pressurized water vapor stretching to 0.5% or less.

  Thus, by controlling the fluctuation rate of the temperature of the heating roller and the water vapor pressure, it is possible to suppress the fluctuation of the draw ratio made to the precursor fiber and the fluctuation of the tow fineness caused by the fluctuation.

  The pressure of water vapor in the pressurized water vapor stretching is preferably 200 kPa / g or more so that the stretching of the heated roller and the characteristics of the pressurized water vapor stretching method appear clearly. The water vapor pressure is preferably adjusted as appropriate in consideration of the treatment time. If the pressure is high, leakage of water vapor may increase. Therefore, it is preferably about 600 kPa / g or less industrially.

  After the drying step, the precursor fiber is cooled to a normal temperature state by passing it through a room temperature roll or the like. The cooled precursor fiber is wound around a bobbin by a winder, or transferred into a can and stored, and used for the production of carbon fiber.

[Baking process]
In the firing step, the precursor fibers are fired to obtain carbon fibers. A baking process consists of a flame-proofing process and a carbonization process, and a graphitization process is provided as needed. The conditions for each treatment in the firing step are not particularly limited, but it is preferable to set conditions under which structural defects such as voids are unlikely to occur inside the fiber.

<Flame resistance treatment>
In the flameproofing treatment, the precursor fiber is heated for an arbitrary time under tension or stretching conditions in an oxidizing atmosphere to form a flameproof fiber. Examples of the flameproofing method include a hot air circulation method and a fixed hot plate method having a porous plate surface.
The heating temperature of the flameproofing treatment is, for example, 200 to 300 ° C.
In flame-resistant treatment is preferably the density of the oxidized fiber is processed until the ^{1.30g / cm 3 ~1.50g / cm 3} .

<Carbonization treatment>
In the carbonization treatment, the carbon fiber is obtained by heating the flame resistant fiber obtained by the flame resistance treatment in an inert gas atmosphere. Carbonization treatment consists of a pre-carbonization operation and a carbonization operation.
The pre-carbonization operation is performed under a tension in an inert gas atmosphere having a maximum temperature of 550 to 800 ° C., and in a temperature range of 300 to 500 ° C., the heating rate is 500 ° C./min or less, preferably 300 ° C./min or less. Then, the flame resistant fiber is heated to be pre-carbonized fiber. This pre-carbonization operation can improve the mechanical properties of the carbon fiber.
As the inert gas, a known inert gas such as nitrogen, argon, helium or the like can be adopted, but nitrogen is desirable in terms of economy.
The carbonization operation is performed by heating the pre-carbonized fiber at a temperature increase rate of 500 ° C./min or less, preferably 300 ° C./min or less in a temperature range of 1000 to 1200 ° C. in an inert atmosphere of 1200 to 3000 ° C. Carbon fiber. This carbonization operation can improve the mechanical properties of the carbon fiber.
The atmospheric gas is the same as that of the pre-carbonization operation.

<Surface treatment>
The obtained carbon fiber is further subjected to a surface treatment to improve the adhesion to the matrix of the composite material. As the surface treatment method, a gas phase or a liquid phase treatment can be used, but electrolytic treatment is preferable from the viewpoint of productivity and variation. As an electrolytic solution used for the electrolytic treatment, an acid such as sulfuric acid, nitric acid or hydrochloric acid, an alkali such as sodium hydroxide, potassium hydroxide or tetraethylammonium hydroxide or a salt thereof can be used, but more preferably contains an ammonium ion. An aqueous solution is preferred. For example, ammonium nitrate, ammonium sulfate, ammonium persulfate, ammonium chloride, ammonium bromide, ammonium dihydrogen phosphate, ammonium dihydrogen phosphate, ammonium hydrogen carbonate, ammonium carbonate, or a mixture thereof can be used.
The amount of electricity in the electrolytic treatment varies depending on the carbon fiber used. For example, the higher the degree of carbonization, the higher the amount of electricity that needs to be supplied.
The obtained carbon fiber is further subjected to sizing treatment as necessary. The sizing agent is preferably a sizing agent having good compatibility with the matrix, and is selected according to the matrix.
By performing these surface treatments, the adhesion between the carbon fibers and the matrix becomes an appropriate level, and thus mechanical properties balanced in the vertical and horizontal directions are exhibited.

(How to find the color difference)
When the carbon fiber precursor acrylic fiber containing an iron element described in this patent is heat-treated, the color becomes darker than that obtained by heat-treating a normal carbon fiber precursor acrylic fiber under the same conditions. The method for obtaining the color difference of the fibers after the heat treatment is not particularly limited. For example, L *, a *, and b * are obtained by a color difference meter, and the color difference = (L * × L * + a * × a * + b * ×). b *) ^Can be obtained by ^1/2 .

  Hereinafter, the present invention will be specifically described by way of examples, but the present invention is not limited thereto.

(Measuring method)
[Content of iron element contained in precursor fiber]
After weighing 2 g of the precursor fiber in a platinum crucible, it is heated on the hot plate until no smoke is produced. Further, it is transferred to a 600 ° C. muffle furnace and incinerated. Add 2 ml of concentrated hydrochloric acid / water (1: 1) to dissolve the ash on a hot plate, and heat until just before concentration and drying. After dissolving in 0.1 mol / l hydrochloric acid aqueous solution, it is made into 10 ml. The amount of iron element is measured by ICP emission spectroscopy.
The ICP emission analysis was measured using a CID high-frequency plasma emission spectrometer (manufactured by Thermo Electron Co., Ltd., model number: IRIS-AP advantage).

[Carbon fiber strand strength, elastic modulus]
The carbon fiber strand strength and elastic modulus were measured according to an epoxy resin-impregnated carbon fiber strand method according to JIS-R-7608. The number of measurements was 10 times, and the average value was used as an evaluation target.

[Color difference]
The target yarn was wound around a 3 cm × 3 cm plate until the plate disappeared to prepare a measurement sample. Next, measurement was performed with SE-2000 manufactured by Nippon Denshoku Co., Ltd. with a light source of C / 2 and a spot size of 0.6 mm. The measurement was performed at n = 7, and two points of the largest value and the smallest value were excluded, and the rest were averaged to obtain L *, a *, and b *. Next, color difference = (L * × L * + a) from the precursor L *, a *, b * and the precursor L *, a *, b *, which are the low iron content and no decrease in strength * × a * + b * × b *) A color difference was obtained by applying the equation of ^1/2 .

[Fiber heat treatment]
In the atmosphere, a hot air circulation type heating furnace in which the wind direction is perpendicular to the precursor fiber bundle and the wind speed is controlled to 0.5 m / sec is used, and groove rolls with a width of 5 mm are installed at the inlet and outlet of the furnace. Thus, the width of the precursor fiber bundle was regulated, and heat treatment was performed for 7 minutes under a treatment temperature of 200 ° C. in a furnace and a load of 1.5 g / tex.

(Reference Example 1)
[Production of acrylonitrile polymer]
The inside of the overflow type polymerization vessel is always 74.75% by mass of deionized water and 25% by mass of monomer (composition ratio is acrylonitrile (AN): acrylamide (AAm): methacrylic acid (MAA) = 96% by mass: 3% by mass: 1% by mass), ammonium persulfate 0.1% by mass, ammonium hydrogen sulfite 0.15% by mass, and ferrous sulfate heptahydrate 2ppm. A suitable amount of sulfuric acid was added so that the temperature in the vessel was maintained, and stirring was performed while maintaining the temperature in the vessel at 50 ° C. The overflowed polymerization slurry was washed and dried to obtain an acrylonitrile-based polymer having AN / AAm / MAA = 96.5 / 2.7 / 0.8 (mass ratio).

[Production of acrylonitrile polymer solution (spinning stock solution)]
21.2% by mass of the acrylonitrile polymer obtained above and 78.8% by mass of dimethylformamide were mixed and dissolved by heating to obtain a spinning dope.

[Production of oil dispersion]
In a PTFE container, 100 parts by mass of amino-modified silicone (manufactured by Shin-Etsu Chemical Co., Ltd., trade name: KF-8002) was mixed with 40 parts by mass of an emulsifier (manufactured by Kao Corporation, product name: Emulgen 108). Thereafter, the mixture was stirred with a homomixer coated with PTFE while adding 25 parts by mass of water to obtain a gel. While stirring this gel with a homomixer, amino-modified silicone was added so as to be 8 equivalents to the emulsifier. Subsequently, water was added while stirring with a homomixer so that water: amino-modified silicone: emulsifier (mass ratio) = 98.65: 1.2: 0.15 to obtain an oil dispersion.

[Precursor fiber production]
The spinning solution was discharged into a coagulation bath composed of an aqueous dimethylacetamide solution having a concentration of 67% by mass and a temperature of 38 ° C. from a spinning nozzle having a pore diameter of 75 μm and a hole number of 6000 to obtain a coagulated yarn. The obtained coagulated yarn was stretched 1.1 times in air, then washed and desolvated while being stretched 3.0 times in hot water. The solvent-removed coagulated yarn was immersed in the oil dispersion and densely dried with a 140 ° C. heating roller. Next, a precursor fiber having a circular cross section with a single fineness of 1.2 dtex was produced at a drawing speed of 100 m / min by stretching 3.0 times in steam at a pressure of 0.22 MPa. The amount of iron element in the produced precursor fiber was 0.4 ppm.

[Manufacture of carbon fiber]
The precursor fiber was passed through a flameproofing furnace having a temperature gradient of 220 to 260 ° C. (flameproofing treatment) and fired in a carbonization furnace having a temperature gradient of 400 to 1300 ° C. in a nitrogen atmosphere (carbonization treatment). Thereafter, electrolytic oxidation treatment and sizing treatment were performed to obtain carbon fibers. The obtained carbon fiber had a strand strength of 5.8 GPa and an elastic modulus of 280 GPa.

Example 1
Manufacture and various measurements of carbon fiber precursors and carbon fibers containing iron in a content that falls within 10% strength reduction.
Carbon fibers were obtained in the same manner as in Reference Example 1 except that iron (III) nitrate was added to the oil dispersion so that the amount of iron element in the oil dispersion was 0.3 × 10 ⁻⁶ g / g. . The amount of iron element in the produced precursor fiber was 0.9 ppm. The obtained carbon fiber had a strand strength of 5.7 GPa, an elastic modulus of 270 GPa, and a color difference of 3.0 when heat-treated with the carbon fiber precursor of Reference Example 1. The strength reduction rate was 1.8%.

(Example 2)
Manufacture and various measurements of carbon fiber precursors and carbon fibers containing iron elements that cause a strength drop of 10% or more.
Carbon fibers were obtained in the same manner as in Reference Example 1 except that iron (III) nitrate was added to the oil dispersion so that the amount of iron element in the oil dispersion was 2.0 × 10 ⁻⁶ g / g. . The amount of iron element in the produced precursor fiber was 6.3 ppm. The obtained carbon fiber had a strand strength of 5.0 GPa and an elastic modulus of 240 GPa. The color difference when heat-treated with the carbon fiber precursor of Reference Example 1 was 12.0. The strength reduction rate was 13.8%.

  From the above results, it is possible to discriminate a carbon fiber precursor acrylic fiber that may undergo strength reduction due to the influence of iron element by performing heat treatment and measuring the color difference. If the value of Example 1 is used, the strength reduction of 1.8% can be discerned, and if the value of Example 2 is used, the strength reduction of 13.8% can be discerned.

Claims (1)

A quality control method for carbon fiber precursor acrylic fibers comprising at least the following steps (1) to (5).
(1) A step of firing carbon fiber precursor acrylic fiber containing less than 0.5 ppm of iron and a carbon fiber precursor acrylic fiber containing more than 0.5 ppm of iron under the same firing conditions to obtain carbon fiber,
(2) The strand strength of each carbon fiber obtained in (1) above is determined, and the strand strength of the carbon fiber obtained by firing the carbon fiber precursor acrylic fiber containing less than 0.5 ppm of the iron content. A step of determining the iron content at which the strength reduction rate of the strand strength of the carbon fiber obtained by firing the carbon fiber precursor acrylic fiber containing 0.5 ppm or more of the iron content is 0.5% or more and 20% or less,
(3) A carbon fiber precursor acrylic fiber containing less than 0.5 ppm of iron and a carbon fiber precursor acrylic fiber containing iron that has a strength reduction rate of 0.5% or more and 20% or less. A step of heat-treating at the same conditions for 1 minute to 20 minutes at a time;
(4) A step of obtaining a color difference of the heat-treated fiber obtained in (3) with a color difference meter,
(5) A step of detecting a process abnormality based on the color difference obtained in (4).