JP7468343B2 - Method for producing flame-retardant fiber bundle and method for producing carbon fiber bundle - Google Patents

Description

The present invention relates to a method for producing a flame-retardant fiber bundle and a method for producing a carbon fiber bundle. More specifically, the present invention relates to a method for producing a flame-retardant fiber bundle used in a carbon fiber bundle, and a method for producing a carbon fiber bundle with high tensile strength obtained by using such a flame-retardant fiber bundle.

Carbon fiber bundles have excellent specific strength and specific elastic modulus, and are therefore used in a wide range of fields, including the aerospace industry, sports applications such as fishing rods and tennis rackets, and general industrial applications such as wind turbine blades and automobiles. In recent years, the demand for carbon fiber bundles has been increasing year by year, not only for aircraft but also for automobiles. Customers are requesting improvements in the quality of carbon fiber bundles, particularly their tensile strength (hereinafter referred to as "strength").

The strength of carbon fiber bundles is affected by the type of precursor fiber bundles that are the raw material for carbon fiber bundles, so polyacrylonitrile precursor fiber bundles are preferably used over pitch-based ones because they are easier to develop strength. Also, one of the factors that affect strength is known to be minute defects present in carbon fiber bundles. Causes of minute defects include scratches and voids in the single fibers that make up the carbon fiber bundles due to contact or adhesion with foreign matter such as dust or metal during the carbon fiber bundle manufacturing process, scratches on the surface of the single fibers due to adhesion between the single fibers, and scratches on the carbon fiber bundle itself caused by rubbing against rollers or slits. Whether such defects occur on the surface or inside of the single fibers of the carbon fiber bundle, the strength of the carbon fiber bundle decreases as the size and number of the defects increase.

Polyacrylonitrile carbon fiber bundles are generally obtained by heating a polyacrylonitrile precursor fiber bundle at 200-300°C in an oxidizing gas atmosphere to obtain a flame-resistant fiber bundle, and then heating it to 1200°C or higher in an inert atmosphere. The process of heat-treating the precursor fiber bundle to obtain a flame-resistant fiber bundle is called the flame-resistant process. Here, an oxidizing gas atmosphere refers to an atmosphere containing a gas that promotes oxidation in order to oxidize the material to be treated. Air containing oxygen is included in the oxidizing gas.

A polyacrylonitrile precursor fiber bundle is usually made up of 1,000 to 60,000 single fibers. In the flame-proofing process, a widely known method is to apply a silicone-based oil to the polyacrylonitrile precursor fiber bundle to prevent the single fibers from fusing together. Silicone-based oil has excellent heat resistance and is effective in preventing the fusing of the single fibers together, but when heated, it is oxidized and decomposed, easily generating fine particles. The generated fine particles then float in the flame-proofing furnace and adhere to the fiber bundle as it remains in the furnace, damaging the fiber bundle and reducing the strength of the final product, the carbon fiber bundle.

In the flame-resistant furnace, the oxidizing gas (typically air) circulated through the circulation flow path by a fan is controlled to keep the temperature inside the furnace constant by a heater installed in the circulation duct and its control mechanism, and the polyacrylonitrile fiber bundles are heat-treated at a predetermined temperature while being folded around multiple rollers inside the furnace.

In the flame-retardant treatment process for polyacrylonitrile fiber bundles, it is known that as hot air circulation is repeated, foreign matter such as fuzz and powder from the strands accumulates in the hot air, contaminating the flame-retardant fiber bundles (Patent Document 1).

To address this issue, Patent Document 1 discloses a method for removing fine particles by collecting them in a flame-resistant furnace with a porous plate, thereby stabilizing the strength of the carbon fiber. Patent Document 2 discloses a method for manufacturing a flame-resistant fiber bundle in which the concentration of fine particles in the oxidizing gas atmosphere, the width of the flame-resistant fiber bundle, the wind speed of the circulating hot air in the flame-resistant furnace, the length of the flame-resistant furnace, and the passage speed of the flame-resistant fiber bundle in the furnace are set within predetermined ranges in order to prevent fine particles such as dust contained in the oxidizing gas circulating in the flame-resistant furnace from adhering to the flame-resistant fiber bundle.

Patent Document 3 discloses a method of suppressing the generation of static electricity by installing conductive fibers in order to solve operational problems such as the generation of static electricity in precursor fiber bundles that are subjected to a flame-retardant process, which are intermediate carbon fiber bundles, but before they are subjected to a preliminary carbonization process, i.e., fiber bundles during the flame-retardant process, which reduces the convergence of the fiber bundles and causes them to wind around rollers or become mixed with adjacent fiber bundles.

In Patent Document 4, the acrylic fiber bundles that are the carbon fiber precursors are non-conductive and become charged by peeling when passing through rolls in the process, or by friction with various guides. Such static electricity makes it easy for surrounding dust to adhere to the surface of the acrylic fiber bundles, raising concerns about a decline in quality, and it also makes them easy for the bundles to wrap around the rolls in the process, causing process problems. Therefore, a method is disclosed for producing acrylic fiber bundles while removing the charged static electricity.

Patent documents 5 and 6 disclose a method of removing static electricity by bringing fibers made conductive by metal plating into contact with a statically charged object to be treated. Patent document 7 discloses a static elimination brush in which the tips of conductive fibers made of metal fibers, carbon fibers, or the like are attached to a holding member made of a conductive material such as stainless steel or aluminum.

JP 2006-57222 A JP 2014-25167 A Japanese Patent Application Laid-Open No. 8-246248 JP 2010-13777 A Japanese Patent Application Laid-Open No. 48-95791 Japanese Utility Model Publication No. 49-9176 Japanese Patent Application Laid-Open No. 3-121009

However, although the invention described in Patent Document 1 describes the removal of foreign matter from inside the flame-resistant furnace using a porous plate such as a wire mesh or punched plate as a foreign matter removal means, it is difficult to completely remove small particles or particles containing volatile adhesive components such as tar.

The invention described in Patent Document 2 describes controlling the concentration of fine particles at a constant level during the flame-resistant process, but it is not possible to remove fine particles that adhere to fiber bundles during the flame-resistant heat treatment or to flame-resistant fiber bundles when they become statically charged.

The invention described in Patent Document 3 describes that, in a manufacturing apparatus for continuous production of carbon fiber, static electricity that is generated when intermediate carbon fiber bundles, which are fiber bundles from the precursor fiber bundle subjected to the flame-proofing or infusibility process to the pre-carbonized fiber bundle, which is the fiber bundle before being supplied to the pre-carbonization process, leave the roller is eliminated by placing a conductive fiber bundle such as a carbon fiber bundle nearby, thereby reducing roller winding. However, there is no description of the effect of fine particles such as dust present in the flame-proofing process on the strength of the polyacrylonitrile-based carbon fiber bundle due to adhesion to the traveling fiber bundle, and there has been no sufficient study on reducing static electricity to a level where fine particles do not adhere to the flame-proofed fiber bundle, and the effect of static elimination on the strength of the carbon fiber bundle is unknown.

The invention described in Patent Document 4 describes the removal of static electricity from polyacrylonitrile precursor fiber bundles, which are the raw material for carbon fiber bundles, but does not describe the removal of static electricity in the sintering process used to manufacture carbon fiber bundles. Since the fiber bundles are constantly exposed to gas containing fine particles generated in the flame-resistant process, even if polyacrylonitrile precursor fiber bundles manufactured by removing static electricity were used, it would not be possible to solve the reduction in strength caused by the adhesion of fine particles to the fiber bundle surface in the flame-resistant process.

In the inventions described in Patent Documents 5 and 6, metal-plated fibers are used as conductive fibers for eliminating static electricity, but there is no description whatsoever of the effect of static electricity on carbon fiber bundles, and sufficient consideration has not been given to reducing static electricity to a level where fine particles do not adhere to the flame-retardant fiber bundles, so the effect of static electricity elimination on the strength of carbon fiber bundles is unknown.

The invention described in Patent Document 7 relates to an anti-static brush using conductive fibers such as carbon fibers, but the objects to be anti-staticized are synthetic resin films such as paper and overhead projector film, which are output media in devices such as printers, copiers, and facsimiles. There is no mention whatsoever of the effect of static electricity on the carbon fiber bundles, and the effect of anti-staticization on the strength of the carbon fiber bundles is unknown.

In the course of investigating a method for preventing fine particles floating in a flame-resistant furnace from adhering to flame-resistant fiber bundles, the inventors discovered that the fine particles floating in the flame-resistant furnace are charged with static electricity and adhere to the fiber bundles passing through the flame-resistant process and the flame-resistant fiber bundles after the flame-resistant process are completed, reducing their strength. That is, the fine particles floating in the flame-resistant furnace are charged and adhere to the flame-resistant fiber bundles by electrostatic interaction. In the manufacture of polyacrylonitrile-based carbon fiber bundles, fine particles such as silica generated when a silicone-based oil is heated and oxidized and decomposed, dust flowing into the flame-resistant furnace from the outside air, and dust derived from tar solidification are likely to be generated in the flame-resistant furnace, and the concentration of the fine particles increases over time, making it easy for the fine particles to adhere to the fiber bundles traveling through the flame-resistant process. Furthermore, the fine particles attached to the flame-resistant carbon fiber bundles are difficult to remove even in the downstream carbonization process, reducing the strength of the carbon fiber bundles.

The problem that the present invention aims to solve is to suppress adhesion of fine particles to the flame-resistant fiber bundle and enable the production of a high-strength carbon fiber bundle by performing flame-resistant heat treatment in an oxidizing atmosphere in which the concentration of fine particles having a particle size of 0.3 μm or more is 300 particles/liter or more while maintaining a low surface potential of the fiber bundle traveling in the initial and/or final stages of the flame-resistant heat treatment.

In order to solve the above problems, the present invention comprises the following configuration.

A method for producing a flame-retardant fiber bundle in which a polyacrylonitrile fiber bundle is flame-retarded at a temperature of 200 to 300°C in an oxidizing atmosphere in which the concentration of fine particles with a particle diameter of 0.3 μm or more is 300 particles/liter or more, is provided, in which when the specific gravity of the fiber bundle is 1.15 to 1.25 and when the specific gravity of the fiber bundle is 1.30 to 1.45 , a conductive fiber bundle having a resistivity of 20×10 ^-4 Ω·cm or less is brought into contact with or close to the fiber bundle, thereby setting the surface potential of the fiber bundle to -1 kV to +1 kV.

Furthermore , the present invention provides a method for producing a flame-resistant fiber bundle using a carbon fiber bundle as the conductive fiber bundle.

The method for producing carbon fiber bundles of the present invention is characterized in that after obtaining a flame-retardant fiber bundle by the method for producing a flame-retardant fiber bundle, the flame-retardant fiber bundle is carbonized in an inert atmosphere at a temperature of 1000 to 2500°C.

In the present invention, when flame-retarding polyacrylonitrile fiber bundles, the amount of fine particles adhering to the fiber bundle can be suppressed to a certain amount or less by setting the surface potential of the traveling fiber bundle to -1 kV to +1 kV. As a result, it becomes possible to carbonize polyacrylonitrile fiber bundles with a certain amount or less of fine particles adhering thereto, and the effect of increasing the strength of the resulting carbon fiber bundle is obtained.

The polyacrylonitrile precursor fiber bundle used as the raw material for the carbon fiber bundle in the present invention can be obtained, for example, by using a homopolymer or copolymer of acrylonitrile as the acrylic polymer and spinning it using an organic or inorganic solvent. The acrylic polymer is a polymer consisting of 90% by mass or more of acrylonitrile, and if necessary, a comonomer copolymerizable with acrylonitrile can be used in an amount of 10% by mass or less. As the comonomer, at least one selected from the group consisting of acrylic acid, methacrylic acid, itaconic acid, and their methyl esters, propyl esters, butyl esters, alkali metal salts, ammonium salts, allylsulfonic acid, methallylsulfonic acid, styrenesulfonic acid, and their alkali metal salts can be used.

There is no particular restriction on the method for producing the polyacrylonitrile precursor fiber bundle, which is the raw material of the polyacrylonitrile carbon fiber bundle of the present invention, but it can be obtained by spinning a homopolymer or copolymer of acrylonitrile as an acrylic polymer using an organic or inorganic solvent. The solvent can be appropriately selected from known solvents such as organic solvents such as dimethylacetamide, dimethylsulfoxide, and dimethylformamide, and aqueous solutions containing inorganic compounds such as nitric acid, zinc chloride, and sodium thiocyanate. The spinning method may be either wet spinning, in which the spinning solution is spun into a solvent in a coagulation bath, or dry-wet spinning, in which the spinning stock solution is spun into air and then coagulated in a bath. After spinning, a polyacrylonitrile precursor fiber bundle can be obtained through processes such as drawing, washing with water, applying an oil agent, drying and densifying, and post-drawing if necessary. When producing a polyacrylonitrile precursor fiber bundle, a silicone-based oil agent may be applied as an oil agent, but it is preferable that at least a part of the silicone-based oil agent applied to the polyacrylonitrile precursor fiber bundle contains amino-modified silicone. These silicone oils may contain surfactants, heat stabilizers, etc. Silicone oils are often used as emulsions, and it is preferable to use an emulsifier in combination. The emulsifier is a compound that has surface activity to promote the formation of emulsion and stabilize it, and a specific example of the emulsifier is preferably polyethylene glycol alkyl ether.

The single yarn fineness of the polyacrylonitrile fiber bundle to which the oil agent has been applied is preferably 0.4 to 1.7 dtex. It is more preferable that the number of single yarns per fiber bundle is 1,000 to 60,000.

The polyacrylonitrile fiber bundle thus obtained is heat-treated at a temperature of 200 to 300°C to perform a flame-resistant treatment. In the method for producing a flame-resistant fiber bundle of the present invention, the polyacrylonitrile fiber bundle is flame-resistant treated in an oxidizing atmosphere to obtain a flame-resistant fiber bundle. Air is preferable as a gas to be used in the oxidizing atmosphere from the viewpoint of cost. When the polyacrylonitrile fiber bundle to which a silicone-based oil agent has been applied is heat-treated in the flame-resistant process, when the silicone-based oil agent applied to the surface of the fiber bundle is heated and oxidized or volatilized, fine particles called silica containing silicon are always formed in the flame-resistant furnace, so that the fine particles such as silica containing silicon increase over time and are present floating in the flame-resistant furnace. That is, the silicone-based oil agent is thermally decomposed in the flame-resistant furnace, and silica is formed, which contaminates the flame-resistant furnace as fine particles together with dust, etc., and if it remains as a foreign matter in the running fiber bundle, it reduces the strength of the carbon fiber bundle finally obtained. The amount of fine particles adhering to the flame-resistant fiber bundle traveling in the flame-resistant furnace increases over time, and the strength of the carbon fiber bundle tends to decrease over time. The present invention is particularly effective in suppressing the decrease in strength of the carbon fiber over time, that is, in maintaining the strength of the carbon fiber bundle at a constant level over time.

When a silicone-based oil is used in the process of obtaining polyacrylonitrile precursor fiber bundles, it is known that silicon derived from the silicone-based oil during carbonization treatment combines with carbon vaporized from the high-temperature furnace structural material, carbon derived from the carbon fiber itself, or nitrogen used as an inert gas to produce various silicon compounds such as silicon carbide and silicon nitride. When these silicon compounds adhere to the fiber bundles, they cause defects in the high-temperature furnace, and when a large amount of silicon compounds accumulate in the high-temperature furnace, they rub against the running carbon fiber bundles and generate fluff, which also reduces the strength of the carbon fiber bundles. For this reason, it is important to suppress the adhesion of fine particles such as silica to the running fiber bundles during the flame-resistant heat treatment in order to prevent a decrease in the strength of the carbon fiber bundles.

As the flame-resistant furnace, a horizontal flame-resistant furnace in which the fiber bundle runs horizontally in a heat treatment chamber in which hot air is circulating is preferably used, but a vertical flame-resistant furnace in which the fiber bundle runs vertically may also be used. At both ends of the inside or outside of the flame-resistant furnace, rollers for folding the fiber bundle are installed in multiple stages, and the fiber bundle that passes along the rollers in the flame-resistant furnace is reversed in its traveling direction by the folding rollers and passes through the flame-resistant furnace repeatedly, and the polyacrylonitrile fiber bundle is flame-resistant treated by circulating hot air vertically or horizontally to the traveling direction of the fiber bundle. A horizontal flame-resistant furnace in which the horizontally crossing fiber bundle is reversed in its traveling direction by the folding rollers is preferred because of the productivity of being able to remove the fiber bundle wrapped around the folding rollers without stopping the machine and the ease of handling the fiber bundle, such as threading and splitting the fiber bundle.

At this time, in order to ensure sufficient strength when the carbon fiber bundle is manufactured, it is preferable that the single yarn fineness of the flame-resistant fiber bundle is 0.4 to 1.7 dtex. The specific gravity of the fiber bundle is 1.15 to 1.25 and/or the specific gravity of the fiber bundle is 1.30 to 1.45. The specific gravity of the polyacrylonitrile fiber bundle increases as the heat treatment in the flame-resistant process progresses. That is, when the specific gravity of the fiber bundle is 1.15 to 1.25, it corresponds to the initial stage of the flame-resistant process in an oxidizing atmosphere, and when the specific gravity of the fiber bundle is 1.30 to 1.45, it corresponds to the final stage of the flame-resistant process and the stage where the heat treatment in the flame-resistant process is completed by passing through a flame-resistant furnace. In other words, the polyacrylonitrile fiber bundle to be destaticized is a fiber bundle that is undergoing a flame-resistant heat treatment in the initial or final stage of the flame-resistant process, or a flame-resistant fiber bundle that has been passed through a flame-resistant furnace after the flame-resistant heat treatment is completed.

Generally, when two objects are rubbed together, positive and negative static electricity is generated at the same time. In other words, static electricity is generated when surface atoms come into contact with each other due to friction between materials. An object that receives electrons through the transfer of electrons becomes negatively charged, while an object that loses electrons becomes positively charged. Static electricity is generated by repeated friction and peeling between two objects, and there is an electrostatic series that indicates whether the object is likely to be charged with positive or negative static electricity. In the electrostatic series of fibers, it is known that acrylic fibers including polyacrylonitrile precursor fiber bundles tend to be charged with negative static electricity compared to other natural and synthetic fibers. This invention makes use of the characteristics of static electricity, which instantly becomes zero when discharged, and is characterized by the fact that the electrostatic charge generated by the repeated friction and peeling of the fiber bundle running through the flame-resistant process against the folding roller is eliminated efficiently and without cost by bringing conductive fibers into close proximity to or into contact with the fiber bundle.

In the flame-resistant furnace, in addition to fine particles such as silica and dust generated by heating and oxidizing the silicone-based oil, fine particles containing metal elements from the outside air and equipment sucked into the flame-resistant furnace from the periphery of the flame-resistant furnace, fine particles derived from tar components generated from the silicone-based oil and polyacrylonitrile-based fiber bundles themselves tend to accumulate in the flame-resistant furnace due to the continuous production of carbon fiber bundles, and these cause a decrease in strength. It is better for the hot air circulating in the flame-resistant furnace to contain fewer fine particles such as the above-mentioned dust present in oxidizing gases such as air, but since such fine particles are constantly generated and accumulated in oxidizing gases, it is extremely difficult industrially to reduce the fine particle concentration to zero. Representative metal elements of fine particles containing metal elements include sodium, magnesium, aluminum, manganese, iron, cobalt, nickel, and zinc. These fine particles and dust adhere to the traveling fiber bundle and form defects on the surface and inside of the single yarns that make up the carbon fiber bundle, causing a decrease in the strength of the carbon fiber bundle. The fine particles having a particle size of 0.3 μm or more as defined in the present invention include fine particles composed of a single substance such as silica, dust particles, tar, and metal fine particles containing metal elements, as well as particulates composed of a combination of multiple of these substances.

On the other hand, the strength level of the obtained carbon fiber bundle can be maintained at a high level by filtering the outside air supplied to the flame-resistant furnace with a high-performance filter, using a rust-resistant material such as stainless steel for the metal parts used in the flame-resistant furnace, and keeping the amount of silicone-based oil used low within the range where the desired physical properties are expressed. The industrial lower limit of the fine particle concentration is generally 300 particles/liter or more for fine particles of 0.3 μm or more. The present invention is even more effective at such a fine particle concentration. The present invention changes the surface potential of the fiber bundle from -1 kV to +1 kV so that the fine particles do not adhere to the fiber bundle due to static electricity when the fiber bundle runs in the flame-resistant furnace where fine particles such as dust are present, so that the adhesion of the fine particles due to static electricity can be suppressed even if fine particles are present. Therefore, even if there are a relatively large number of fine particles in the flame-resistant furnace, the fiber bundle runs in a state where the fine particles are difficult to adhere to the fiber bundle, and the strength of the carbon fiber bundle is higher than when the surface potential is not controlled. However, if an excessive amount of fine particles are present in the flame-resistant furnace, the fine particles will naturally adhere to the fiber bundle even when the fiber bundle is not charged with static electricity, i.e., when the fiber bundle is de-staticized, resulting in a decrease in strength. Therefore, although there is no particular upper limit on the concentration of fine particles having a particle size of 0.3 μm or more, it is preferable that the upper limit be 10,000 particles/L or less.

The present invention is a method for flame-retarding polyacrylonitrile fiber bundles at temperatures of 200 to 300°C in an oxidizing atmosphere while controlling the surface potential, which is a basic static electricity characteristic of the traveling fiber bundle, to -1 kV to +1 kV. In particular, when the specific gravity of the traveling fiber bundle is 1.15 to 1.25 and/or when the specific gravity of the fiber bundle is 1.30 to 1.45, that is, when the surface potential of the polyacrylonitrile fiber bundle, which is a flame-retardant fiber bundle during or after the flame-retardant treatment, needs to be controlled to -1 kV to +1 kV. If there is a lot of static electricity, i.e., if the surface potential is high, the force caused by the static electricity acts and the traveling fiber bundle is attracted to the roller, causing winding and fluffing, which not only reduces both the quality and processability, but also damages the fiber bundle itself, reducing the toughness of the fiber bundle itself. Furthermore, dust and fine particles around the fiber bundle that is running in a statically charged state adhere to the fiber bundle, causing scratches on the single yarn due to abrasion caused by contact with the roller. Furthermore, defects may be formed due to the adhered fine particles during high temperature treatment in the pre-carbonization process and carbonization process.

If the traveling fiber bundle is thus charged with static electricity, the strength of the carbon fiber bundle will decrease for both the fiber bundle and the single yarns that make up the fiber bundle, and so reducing the surface potential of the fiber bundle as much as possible is extremely important in order to achieve high strength for the carbon fiber bundle. The best state is one in which the surface potential is zero, i.e., uncharged, but since the traveling fiber bundle comes into contact with and rubs against the roller and is constantly generating static electricity, industrially it is preferable for the potential to be in the range of -1 kV to +1 kV.

In the present invention, the method of setting the surface potential of the fiber bundle to -1 kV to +1 kV, i.e., the method of eliminating static electricity from the fiber bundle, includes, but is not limited to, a contact type using a conductive fiber bundle and a non-contact type. A contact type static elimination method is to directly contact a conductive fiber bundle with a running fiber bundle. A non-contact type static elimination method is to place a conductive fiber bundle in close proximity to the running fiber bundle.

Common methods for removing static electricity include using a voltage-applied static electricity remover or using the self-discharge of charged static electricity through a conductor. Using a static electricity remover is cost-intensive, as it requires installation costs. When charged static electricity self-discharges through a conductor, the larger the surface area of the conductor, the greater the amount of discharge. In the case of fiber bundles, the larger the surface area is, the more static electricity is likely to be discharged, and the fiber bundle may be affected by the discharge. Another method is to apply water to the traveling fiber bundle or roller to promote discharge, but surrounding dust and fine particles are adsorbed by the sprayed water, and when this water adheres to the fiber bundle, the dust and fine particles contained in the water may cause a problem of a decrease in the strength of the carbon fiber bundle.

The method of de-electrifying a statically charged fiber bundle using a conductive fiber bundle can efficiently de-electrify the fiber bundle because the single yarns that make up the fiber bundle are close to or in contact with each other. The conductive fiber bundle can be easily arranged, for example, by placing it on a grounded metal roller stand via a metal fastener. Furthermore, if the conductive fiber bundle is damaged, it can be easily removed and replaced with a new conductive fiber bundle. In this way, the de-electrification method using the conductive fiber bundle of the present invention is superior to conventional de-electrification methods in terms of both cost and ease of handling.

The conductive fiber bundles used to remove static electricity may be metal fibers, but they can cause thread breakage and fuzzing when they come into contact with each other and rub against each other, some of the metal components can adhere to the fiber bundles and form defects that reduce their strength, and if they get mixed into a high-temperature furnace such as a carbonization furnace, they can become impurities and cause a decrease in strength. Therefore, using carbon fiber bundles as the conductive fiber bundles is more preferable in that it does not cause contamination. Furthermore, materials of the same type are close in the triboelectric series, and are preferable because they can suppress the generation of new static electricity even when they are removed while in contact. The triboelectric series is an index that indicates whether two substances are more likely to be charged positively or negatively when they are in contact with each other.

The conductive fiber bundle only needs to be of a moderate size, and a minimum of about 6,000 single threads is sufficient for the conductive fiber bundle. Either a carbon fiber bundle consisting of multiple carbon fiber bundles with a small number of filaments or a carbon fiber bundle with a large number of filaments can be used. There is no practical upper limit to the number of filaments in the conductive fiber bundle, but 60,000 will suffice. When using carbon fiber bundles as conductive fiber bundles, the form can be such that static electricity can be eliminated from the running fiber bundle, and in addition to fiber bundles, examples of the form include ropes, brushes, strings, knitted fabrics, and woven fabrics, and are not particularly limited.

The conductive fiber bundle may be placed at one location or at multiple locations spaced apart from each other as long as the static electricity of the traveling fiber bundle can be eliminated. In particular, the conductive fiber bundle is highly effective when placed at a location where the fiber bundle, which is prone to static electricity due to peeling or friction between the fiber bundle and the roller or guide, leaves the roller. The conductive fiber bundle is placed at a location where the distance from the point where the fiber bundle leaves the roller in the traveling direction of the fiber bundle is 0 to 30 cm, preferably 0 to 10 cm. In addition, in the direction perpendicular to the traveling direction of the fiber bundle, the closer the distance between the traveling fiber bundle and the tip of the conductive fiber bundle is, the easier the fiber bundle is to be eliminated and the surface potential can be reduced, so the distance between the fiber bundle and the tip of the conductive fiber bundle is 0 to 50 mm, preferably 0 to 25 mm. In the present invention, the proximity of the conductive fiber bundle to the fiber bundle means that the distance between the fiber bundle and the tip of the conductive fiber bundle is preferably 0 to 50 mm, more preferably 0 to 25 mm.

In the present invention, it is preferable to use a conductive fiber bundle as a means for removing static electricity from a traveling fiber bundle. A conductive fiber bundle is a fiber bundle that has been given electrical conductivity, and is effective in preventing dust adhesion and discharging static electricity. Conductive fiber bundles include synthetic fibers in which conductive materials such as metal, carbon, and graphite are uniformly dispersed or generated by chemical reaction, metal fibers made by fiberizing metals such as stainless steel, copper, aluminum, iron, nickel, and titanium, fibers with a metal coating on the surface, fibers with a resin coating on the surface containing a conductive material, carbon fibers, and metal-coated carbon fibers.

The advantage of using conductive fiber bundles as a means of eliminating static electricity is that, unlike metal wires such as wires and metal wires, the single threads that make up the conductive fiber bundle are always in contact with the running fiber bundle at multiple points, making them highly effective at eliminating static electricity. Another advantage is that even if some of the single threads that make up the conductive fiber bundle break, the remaining single threads that are not broken can continue to have an electrostatic elimination effect, and the conductive fiber bundle can always change shape to match the shape of the running fiber bundle that is charged with static electricity so that it can make contact.

In order to eliminate static electricity from a traveling fiber bundle, the conductive fiber should have sufficient conductivity, and a representative indicator of conductivity is resistivity, which is electrical resistivity. Specifically, the conductive fiber bundle preferably has a resistivity of 20×10 ⁻⁴ Ω·cm or less. If the conductive fiber bundle has such a resistivity, static electricity from the traveling fiber bundle can be eliminated suitably. There is no particular limit to the lower limit of the resistivity.

As the flame-proofing process is an environment in which many of the aforementioned fine particles are present, it is preferable to install the conductive fiber bundles, which are the static elimination means, throughout the entire flame-proofing process, but more preferably, to install the polyacrylonitrile fiber bundles at a position where the specific gravity of the fiber bundle running through the flame-proofing furnace is 1.15 to 1.25 and/or at a position where the specific gravity of the fiber bundle is 1.30 to 1.45. The fiber bundles running during the flame-proofing heat treatment are repeatedly rubbed against and peeled off from the rollers, so static electricity is generated throughout the flame-proofing process, but the generation of static electricity is significant in the early and final stages of flame-proofing, as described below, and therefore the effects of the present invention are particularly great.

This is because the polyacrylonitrile precursor fiber bundles, which are the raw material for carbon fiber bundles, are non-conductive and easily charged with static electricity, and because the fiber bundles have a large thermal shrinkage in the early stages of the flame-resistant heat treatment, they are constantly rubbed against and peeled off from the roller, which makes it easy for static electricity to be generated. Therefore, if a conductive fiber bundle is placed in contact with or close to a fiber bundle with a specific gravity of 1.15 to 1.25, which corresponds to the early stage of the flame-resistant process, a large static electricity removal effect is achieved. In addition, in the fiber bundles at the end of the flame-resistant process or after passing through a flame-resistant furnace, the fiber bundles themselves become weak and the tension of the fiber bundles increases due to the accumulation of mechanical loss caused by the return rollers in the flame-resistant process, so static electricity is likely to be generated when the fiber bundles rub against the rollers, and even if a conductive fiber bundle is placed in contact with or close to a fiber bundle with a specific gravity of 1.30 to 1.45, which corresponds to the final stage of the flame-resistant process or after passing through a flame-resistant furnace, a large static electricity removal effect is achieved.

The flame-retardant fiber bundle thus obtained is pre-carbonized in an inert atmosphere such as nitrogen at a temperature of 300 to 1000°C, and then carbonized in an inert atmosphere such as nitrogen at a temperature of 1000 to 2500°C to obtain a carbon fiber bundle.

After the carbonization treatment, an oxidation surface treatment is performed to generate functional groups on the surface of the carbon fiber bundle to enhance adhesion to the matrix resin. The oxidation surface treatment method includes liquid-phase oxidation using a chemical solution, electrolytic surface treatment in which the carbon fiber is surface-treated in an electrolyte solution, and gas-phase oxidation surface treatment using plasma treatment, among others. The electrolytic surface treatment method is preferably used because it is relatively easy to handle and advantageous in terms of manufacturing costs. The electrolyte solution used in the electrolytic surface treatment can be either an acidic or alkaline aqueous solution, but the acidic aqueous solution is preferably sulfuric acid or nitric acid, which shows strong acidity, and the alkaline aqueous solution is preferably an aqueous solution of an inorganic alkali such as ammonium carbonate, ammonium hydrogen carbonate, or ammonium bicarbonate. The carbon fiber bundle that has been subjected to such an electrolytic surface treatment is washed with water as necessary, and then the moisture is evaporated in a dryer, after which a sizing agent is applied. The type of sizing agent is not particularly limited, but the sizing agent can be appropriately selected from bisphenol A type epoxy resins and polyurethane resins, which are mainly composed of epoxy resins, according to the matrix resin used in the advanced processing.

When the flame-retardant fiber bundle obtained in this manner is used to manufacture carbon fiber bundles, carbon fiber bundles with high strength can be obtained.

The following examples and comparative examples of the present invention are described in more detail. The evaluation and measurement methods for each characteristic were as described below.

<Measurement of particle concentration>
The concentration of particles with a particle size of 0.3 μm or more was measured using a light scattering particle counter (e.g., KC-01E, manufactured by RION Co., Ltd.). Sample air, which is an oxidizing gas, was sucked from the flame-resistant furnace at 0.5 liters/minute for 34 seconds, and the number of particles with a particle size of 0.3 μm or more contained in 0.283 liters was counted, and the value converted into the number of particles per liter was used as the particle concentration (particles/liter).

<Specific gravity of fiber bundle>
The specific gravity of the fiber bundle was measured according to the method described in JIS R7601 (2006). The measurement was performed using the fiber bundle for measuring the surface potential. Ethanol (special grade manufactured by Wako Pure Chemical Industries, Ltd.) was used as the reagent without purification. 1.0 to 1.5 g of fiber bundle was collected and completely dried at 120°C for 2 hours. After measuring the completely dried mass (A), the fiber bundle was immersed in ethanol of known specific gravity (specific gravity ρ) and the mass of the fiber bundle in the ethanol (B) was measured. The specific gravity was calculated according to the following formula.
Specific gravity = (A x ρ)/(A-B).

<Surface potential of fiber bundle>
The surface potential of the fiber bundle was measured at a position 10 cm away in the running direction of the fiber bundle from the point where the fiber bundle separated from the folding back roller in the flame retardant process and at a position 2.5 cm away from the fiber bundle in the direction perpendicular to the fiber bundle, using a non-contact static electricity meter FMX-003 manufactured by SIMCO JAPAN Co., Ltd., to measure the surface potential, which is the static electricity of the fiber bundle.

<Strength of carbon fiber bundle>
The strength was determined according to the following procedure in accordance with the carbon fiber tensile property test method of JIS R7608 (2007). The resin formulation used was "Celloxide (registered trademark)" 2021P (manufactured by Daicel Chemical Industries, Ltd.)/boron trifluoride monoethylamine (manufactured by Tokyo Chemical Industry Co., Ltd.)/acetone = 100/3/4 (parts by mass), and the curing conditions were normal pressure, temperature of 125°C, and time of 30 minutes. Five carbon fiber bundles were measured, and the average value was taken as the strength of the carbon fiber bundle.

<Specific resistance of conductive fiber bundle>
When the electrical resistance per 1 m of the conductive fiber bundle is R (Ω/m), the basis weight per 1 m is Y (g/m), and the density is D (g/cm ³ ), the conductive resistivity was calculated using the following formula.

Specific resistance (×10 ⁻⁴ Ω·cm)=R×Y÷D.

[Example 1]
A spinning dope was prepared from an acrylic polymer, and the spinning dope was coagulated by a dry-wet spinning method. The obtained coagulated yarn was washed with water, stretched, oiled, dried, and stretched with steam to obtain a polyacrylonitrile precursor fiber bundle having a single fiber fineness of 1.1 dtex and a single fiber count of 12,000.

The silicone-based oil used was made of amino-modified silicone, and an amino-modified silicone emulsion containing polyethylene glycol alkyl ether was used as the emulsifier.

Next, in a horizontal hot air circulation type flame-resistant furnace in which the concentration of fine particles with a particle size of 0.3 μm or more in the furnace was 2500 particles/liter and the furnace temperature was 220 to 270° C., a fiber bundle having a specific gravity of 1.18 that had been heat-treated for flame-resistant treatment in an oxidizing atmosphere of air was contacted with a carbon fiber bundle having a resistivity of 15×10 ⁻⁴ Ω·cm as a conductive fiber at a position 10 cm from the point where the fiber bundle left the roller, and the surface potential of the fiber bundle was −0.2 kV (that is, the measurement result of the surface potential after static electricity removal). For reference, the surface potential of the fiber bundle before static electricity removal was −5.0 kV.

The flame-retardant fiber bundle obtained by the flame-retardant treatment was then carbonized at a maximum carbonization temperature of 1350°C in an inert atmosphere, and a sizing agent was applied after surface treatment to produce a carbon fiber bundle. The strength of the obtained carbon fiber bundle was 540 kgf/ ^mm2 (5.3 GPa). The results are shown in Table 1.

[Example 2]
A carbon fiber bundle was obtained in the same manner as in Example 1, except that the carbon fiber bundle, which was a conductive fiber, was placed in close proximity to the running fiber bundle at a position 2 cm away in the vertical direction. The surface potential of the fiber bundle after static elimination was -1.0 kV, and the strength of the obtained carbon fiber bundle was 520 kgf/mm ² (5.1 GPa). The results are shown in Table 1.

[Example 3]
A carbon fiber bundle was obtained in the same manner as in Example 1, except that the specific gravity of the fiber bundle that had been heat-treated for flame retardancy was 1.45. The surface potential of the fiber bundle after static elimination was +0.5 kV, and the strength of the obtained carbon fiber bundle was 520 kgf/ ^mm2 (5.1 GPa). The results are shown in Table 1.

[Example 4]
Carbon fiber bundles were obtained in the same manner as in Example 1, except that when the specific gravity of the fiber bundle that had been heat-treated for flame retardancy reached 1.18 and when the specific gravity of the fiber bundle reached 1.45, a carbon fiber bundle having a resistivity of 15×10 ⁻⁴ Ω·cm as a conductive fiber was brought into contact with each of the traveling fiber bundles at a position 10 cm from the point where the fiber bundle leaves the roller. The surface potentials of the fiber bundles after static elimination were −0.2 kV and +0.2 kV, respectively, and the strength of the obtained carbon fiber bundle was 550 kgf/mm ² (5.4 GPa). The results are shown in Table 1.

[Example 5]
A carbon fiber bundle was obtained in the same manner as in Example 1, except that nickel-coated carbon fiber strands "Tenax (registered trademark)" HTS40 MC (product name) manufactured by Teijin Limited were used as the conductive fibers. Because of the nickel coating, the resistivity was reduced to 0.75×10 ^-4 Ω·cm, and the static elimination effect was enhanced, so the surface potential of the fiber bundle after static elimination was 0 kV. However, the nickel-coated carbon fiber was always in contact with the fiber bundle, and the fiber bundle was constantly rubbing against the nickel, causing damage to the single yarn, and at the same time, the nickel attached to the fiber bundle formed defects in the carbonization process, so the strength of the obtained carbon fiber bundle was 520 kgf/mm ² (5.1 GPa). The results are shown in Table 1.

[Example 6]
A carbon fiber bundle was obtained in the same manner as in Example 1, except that the concentration of fine particles having a particle size of 0.3 μm or more in the flame-resistant furnace was set to 320 particles/liter. The surface potential of the fiber bundle after static elimination was -0.2 kV. The inside of the flame-resistant furnace was clean, and dust and fine particles were unlikely to adhere to the fiber bundle, and the strength of the obtained carbon fiber bundle was 550 kgf/mm ² (5.4 GPa). The results are shown in Table 1.

[Example 7]
A carbon fiber bundle was obtained in the same manner as in Example 1, except that the concentration of fine particles having a particle size of 0.3 μm or more in the flame-resistant furnace was set to 5,000 particles/liter. The surface potential of the fiber bundle after static elimination was −0.2 kV. The strength of the obtained carbon fiber bundle was 520 kgf/mm ² (5.1 GPa). The results are shown in Table 1.

[Example 8]
A carbon fiber bundle was obtained in the same manner as in Example 1, except that the concentration of fine particles having a particle size of 0.3 μm or more in the flame-resistant furnace was set to 8,000 particles/L. The surface potential of the fiber bundle after static elimination was −0.2 kV. The strength of the obtained carbon fiber bundle was 510 kgf/mm ² (5.0 GPa). The results are shown in Table 1.

[Example 9]
A carbon fiber bundle was obtained in the same manner as in Example 1, except that a wire with a diameter of 3 mm made of iron was used. The specific resistance of the wire was reduced to 0.097×10 ⁻⁴ Ω·cm, increasing the static elimination effect, and the surface potential of the fiber bundle after static elimination was 0 kV. However, unlike the conductive fiber bundle, the wire was constantly rubbed in a state of point contact with the fiber bundle, causing damage to the single yarn, and at the same time, the iron attached to the fiber bundle formed defects during the carbonization process, resulting in a strength of the obtained carbon fiber bundle of 510 kgf/mm ² (5.0 GPa). The results are shown in Table 1.

[Example 10]
A carbon fiber bundle was obtained in the same manner as in Example 1, except that the specific gravity of the fiber bundle that had been heat-treated for flame retardancy was 1.23. The surface potential of the fiber bundle after static elimination was −0.2 kV, and the strength of the obtained carbon fiber bundle was 530 kgf/mm ² (5.2 GPa). The results are shown in Table 1.

[Example 11]
A carbon fiber bundle was obtained in the same manner as in Example 1, except that the specific gravity of the fiber bundle that had been heat-treated for flame retardancy was 1.33. The surface potential of the fiber bundle after static elimination was +0.4 kV, and the strength of the obtained carbon fiber bundle was 520 kgf/ ^mm2 (5.1 GPa). The results are shown in Table 1.

[Comparative Example 1]
When the same procedure as in Example 1 was carried out except that no conductive fiber bundle was used, the surface potential rose to -5.0 kV due to the generation of static electricity. When the traveling fiber bundle leaves the roller, the single yarns constituting the fiber bundle are always attracted to the roller due to static electricity, and since there is a lot of dust and fine particles such as silica in the flame-resistant furnace, fine silica particles adhere to the traveling fiber bundle, and therefore, a large number of particulate white silica particles were observed adhering to the flame-resistant fiber bundle after passing through the flame-resistant furnace. The strength of the obtained carbon fiber bundle was reduced to 480 kgf/mm ² (4.7 GPa). The results are shown in Table 1.

[Comparative Example 2]
When the same procedure as in Example 7 was carried out except that no conductive fiber bundle was used, the surface potential rose to -5.0 kV due to the generation of static electricity. When the traveling fiber bundle leaves the roller, the single yarns constituting the fiber bundle are always attracted to the roller due to static electricity, and since there is a lot of dust and fine particles such as silica in the flame-resistant furnace, fine silica particles adhere to the traveling fiber bundle, and therefore, a large number of particulate white silica particles were observed adhering to the flame-resistant fiber bundle after passing through the flame-resistant furnace. The strength of the obtained carbon fiber bundle was reduced to 470 kgf/mm ² (4.6 GPa). The results are shown in Table 1.

[Comparative Example 3]
When the same procedure as in Example 8 was carried out except that no conductive fiber bundle was used, the surface potential rose to -5.0 kV due to the generation of static electricity. When the traveling fiber bundle leaves the roller, the single yarns constituting the fiber bundle are always attracted to the roller due to static electricity, and since there is a lot of dust and fine particles such as silica in the flame-resistant furnace, fine silica particles adhere to the traveling fiber bundle, and therefore, a large number of particulate white silica particles were observed adhering to the flame-resistant fiber bundle after passing through the flame-resistant furnace. The strength of the obtained carbon fiber bundle was reduced to 460 kgf/mm ² (4.5 GPa). The results are shown in Table 1.

[Comparative Example 4]
A carbon fiber bundle was obtained in the same manner as in Example 1, except that the carbon fiber bundle, which was a conductive fiber bundle, was placed in close proximity to the running fiber bundle at a position 10 cm away in the vertical direction. Since the neutralization effect of the conductive fiber bundle was reduced, the surface potential of the fiber bundle after neutralization was high at -2.0 kV, and neutralization was insufficient. The strength of the obtained carbon fiber bundle was 490 kgf/mm ² (4.8 GPa). The results are shown in Table 1.

[Comparative Example 5]
A carbon fiber bundle was obtained in the same manner as in Example 1, except that the flame-resistant heat-treated fiber bundle was placed in close proximity to a position 10 cm away in the vertical direction from the fiber bundle with a specific gravity of 1.45. Since the static elimination effect of the conductive fiber bundle was reduced, the surface potential of the fiber bundle after static elimination was high at +2.0 kV, and static elimination was insufficient. The strength of the obtained carbon fiber bundle was 490 kgf/mm ² (4.8 GPa). The results are shown in Table 1.

[Comparative Example 6]
A carbon fiber bundle was obtained in the same manner as in Example 1, except that the specific gravity of the fiber bundle in contact with the conductive fiber bundle was 1.50. As the specific gravity increased, the fiber bundle became brittle and was more likely to be charged with static electricity due to friction with the roller, and the surface potential after static elimination rose to +3.0 kV, indicating insufficient static elimination. The strength of the obtained carbon fiber bundle decreased to 480 kgf/mm ² (4.7 GPa). The results are shown in Table 1.

[Comparative Example 7]
A carbon fiber bundle was obtained in the same manner as in Example 1, except that the specific gravity of the fiber bundle in contact with the conductive fiber bundle was 1.28. The area where the fiber bundle having such a specific gravity runs is a place where static electricity is relatively unlikely to occur during the flame-proofing process, so the static elimination effect of the conductive fiber bundle was limited, and the surface potential after static elimination was +1.2 kV. The strength of the obtained carbon fiber bundle was reduced to 490 kgf/mm ² (4.8 GPa). The results are shown in Table 1.

Claims (5)

A method for producing a flame-retardant fiber bundle by subjecting a polyacrylonitrile fiber bundle to a flame-retardant treatment at a temperature of 200 to 300°C in an oxidizing atmosphere, the method comprising the steps of: bringing a conductive fiber bundle having a resistivity of 20×10 ⁻⁴ Ω·cm or less into contact with or in close proximity to the fiber bundle when the specific gravity of the fiber bundle is 1.15 to 1.25; and setting the surface potential of the fiber bundle to -1 kV to +1 kV. A method for producing a flame-retardant fiber bundle, comprising flame-retarding a polyacrylonitrile fiber bundle at a temperature of 200 to 300°C in an oxidizing atmosphere in which the concentration of fine particles having a particle diameter of 0.3 μm or more is 300 particles/L or more, and wherein, when the specific gravity of the fiber bundle is 1.15 to 1.25, a conductive fiber bundle having a resistivity of 20×10 ^-4 Ω·cm or less is brought into contact with or in close proximity to the fiber bundle, thereby setting the surface potential of the fiber bundle to -1 kV to +1 kV. The method for producing an oxidation-resistant fiber bundle according to claim 1 or 2 , wherein the conductive fiber bundle is a carbon fiber bundle. The method for producing an oxidation-resistant fiber bundle according to claim 3 , wherein the carbon fiber bundles are made of the same type of material . A method for producing a carbon fiber bundle, comprising obtaining a flame-resistant fiber bundle by the method for producing a flame-resistant fiber bundle according to any one of claims 1 to 4, and then carbonizing the flame-resistant fiber bundle in an inert atmosphere at a temperature of 1000 to 2500°C.