JP7697454B2 - Flame-resistant fiber bundle, method for producing carbon fiber bundle, and flame-resistant furnace - Google Patents

Description

The present invention relates to a flame-retardant fiber bundle and a method for producing a carbon fiber bundle. More specifically, the present invention relates to a flame-retardant fiber bundle and a method for producing a carbon fiber bundle, which can efficiently produce a high-quality flame-retardant fiber bundle, and a flame-retardant furnace.

Carbon fiber has excellent specific strength, specific elastic modulus, heat resistance, and chemical resistance, making it useful as a reinforcing material for various materials, and is used in a wide range of fields, including aerospace, leisure, and general industrial applications.

Generally, a method for producing a carbon fiber bundle from an acrylic fiber bundle is known in which an acrylic fiber bundle in which several thousand to tens of thousands of single fibers of an acrylic polymer are aligned is fed into a flame-resistant furnace, and heat-treated (flame-resistant treatment) by exposing the bundle to hot air of an oxidizing gas such as air heated to 200 to 300° C. supplied from a heated gas supply nozzle (hereinafter simply referred to as a supply nozzle) installed inside the furnace, and then the flame-resistant fiber bundle obtained is fed into a carbonization furnace, and heat-treated (pre-carbonization treatment) in an inert gas atmosphere at 300 to 1,000° C., and then further heat-treated (carbonization treatment) in a carbonization furnace filled with an inert gas atmosphere at 1,000° C. or higher. In addition, the flame-resistant fiber bundle, which is an intermediate material, is also widely used as a material for flame-retardant woven fabrics, taking advantage of its flame-retardant property.

Among the carbon fiber bundle manufacturing processes, the flame-stabilizing process takes the longest processing time and consumes the most energy, so improving the productivity of the flame-stabilizing process is the most important factor in the manufacture of carbon fiber bundles.

In the flame-proofing process, in order to enable a long-term heat treatment, the apparatus for flame-proofing (hereinafter referred to as a flame-proofing furnace) generally has a structure in which the acrylic fiber is turned around a return roller arranged outside the furnace body of the flame-proofing furnace, thereby making the acrylic fiber go back and forth in the furnace body of the flame-proofing furnace many times in the horizontal direction. In the furnace body of such a flame-proofing furnace, a method of supplying hot air in a direction approximately parallel to the running direction of the acrylic fiber bundle is generally called a parallel flow method, and a method of supplying hot air in a direction perpendicular to the running direction of the acrylic fiber bundle is generally called a cross flow method. The parallel flow method includes an end-to-end (hereinafter referred to as ETE) hot air method in which a supply nozzle is installed at the end of the parallel flow furnace and an exhaust nozzle for the gas inside the furnace (hereinafter referred to as exhaust nozzle) is installed at the opposite end, and a center-to-end (hereinafter referred to as CTE) hot air method in which a supply nozzle is installed at the center of the parallel flow furnace and exhaust nozzles are installed at both ends of the supply nozzle. In general, the ETE hot air method requires less equipment cost than the CTE hot air method.

Among the methods for improving productivity in the flame-resistant process, it is effective to increase the density of the acrylic fiber bundles in the furnace body of the flame-resistant furnace by simultaneously transporting a large number of acrylic fiber bundles. Also, it is effective to increase the running speed of the acrylic fiber bundles. However, as the mass per unit volume of the acrylic fiber bundles supplied to the furnace body increases, the amount of heat per unit volume required for heating/removing heat from the acrylic fiber bundles also increases, making it difficult to control the temperature and resulting in a deterioration in the quality of the flame-resistant fiber.

Furthermore, when the running speed of the acrylic fiber bundle is increased, the size of the flame-resistant furnace must be increased to obtain the same amount of heat treatment. In particular, when the size in the height direction is increased, it is necessary to divide the building into multiple stories or increase the load-bearing capacity per unit area of the floor surface, which leads to an increase in equipment costs. Therefore, in order to increase the size of the flame-resistant furnace while suppressing the increase in equipment costs, it is effective to reduce the size in the height direction by increasing the distance per horizontal pass (hereinafter referred to as the flame-resistant furnace length). However, when the flame-resistant furnace length is increased, the heat treatment length is correspondingly increased, making it difficult to control the temperature of the acrylic fiber bundle. This is particularly noticeable in the case of the ETE hot air method.

Therefore, in order to improve the productivity in the flame-proofing process using the ETE hot air method, there was a problem that it was necessary to improve the efficiency of the heating performance and heat removal performance of the acrylic fiber bundle traveling inside the furnace body of the flame-proofing furnace.

In order to solve this problem, Patent Document 1 describes a method in which the discharge surface of the discharge nozzle is spaced apart from the heat treatment chamber and hot air is sucked in from within the heat treatment chamber to form a flow in the gap between the discharge nozzles, thereby making it easier to heat and remove heat from the acrylic fiber bundle.

Furthermore, Patent Document 2 describes a heat treatment method in which, in a CTE hot air method, hot air is supplied to the space between supply nozzles located in the center of the furnace body, thereby making the temperature of the space between the supply nozzles equal to the temperature of the space inside the furnace body.

Furthermore, Patent Document 3 describes a method for improving the sealing performance of a flame-resistant furnace, in which a heating means having a supply surface for blowing out hot air is provided in a flow path gap where the acrylic fiber bundle outside the furnace body enters the furnace body in order to heat the acrylic fiber bundle.

Patent No. 5856081 Patent No. 5856082 Patent No. 4796467

However, according to the findings of the present inventors, the airflow pattern in the furnace body of the flame-proofing furnace cannot be changed by only controlling the flow formed in the gap between the exhaust nozzles in Patent Document 1, and the heating and heat removal effect of the acrylic fiber bundle may not be sufficiently obtained. This is considered to be because the airflow pattern (speed and direction of hot air) near the acrylic fiber bundle in the flame-proofing furnace dominates the heat transfer to the acrylic fiber bundle, and therefore the airflow control by the exhaust nozzles alone may only produce the effect between the exhaust nozzles and may not produce a sufficient effect.

In addition, in Patent Document 2, by supplying hot air between the supply nozzles, airflow turbulence occurs when the hot air crosses the acrylic fiber bundle, so that even at a low air speed, the yarn sway of the acrylic fiber bundle becomes large, and as a result, contact between adjacent acrylic fiber bundles, intermixing of the acrylic fiber bundles, yarn breakage, etc. may occur. In addition, Patent Document 2 only describes the uniformity of the airflow temperature between the supply nozzles and in the space inside the furnace body, and does not disclose controlling the temperature of the acrylic fiber bundle inside the furnace body. That is, the parameters required for temperature control of the acrylic fiber bundle inside the furnace body include the temperature and speed of the hot air, and although there is a description regarding the former temperature, there is no detailed description regarding the speed of the hot air, and there are cases where the temperature of the acrylic fiber bundle cannot be controlled. In addition, it is limited to a parallel flow type CTE hot air method, and there is no specific description regarding the ETE hot air method, which has low equipment costs.

Furthermore, in Patent Document 3, since the hot air supply surface is provided outside the furnace body of the flame-resistant furnace, it may be insufficient to improve the heating and heat removal properties of the acrylic fiber bundle traveling inside the furnace body of the flame-resistant furnace. Also, since the purpose of Patent Document 3 is to improve the sealing property of the flame-resistant furnace, the hot air supply direction is outside the furnace body, and the hot air supplied from the supply surface is blown out of the furnace body as it is, so that an air flow may not be formed between the nozzles through which the acrylic fiber bundle passes.

The method for producing a flame-resistant fiber bundle of the present invention for solving the above problems has the following configuration: That is, it is a method for producing a flame-resistant fiber bundle by heat-treating an aligned acrylic fiber bundle in an oxidizing atmosphere while folding back the aligned acrylic fiber bundle with guide rollers installed on both ends outside the furnace body of a hot-air heating type flame-resistant furnace, in which hot air is supplied from a supply surface provided above and/or below the acrylic fiber bundle in a fiber bundle passing passage above and/or below a supply nozzle for supplying hot air into a heat treatment chamber, the supply nozzle being arranged at one end in the running direction of the acrylic fiber bundle, and the wind speed Vf in the fiber bundle passing passage in a direction approximately parallel to the running direction of the acrylic fiber bundle and the wind speed V in the heat treatment chamber in a direction approximately parallel to the running direction of the acrylic fiber bundle satisfy conditions (1) and (2).
(1) 1.5m/s ≦ Vf ≦ 15m/s
(2) 1.5m/s≦V≦10m/s.

In the method for producing an oxidized fiber bundle of the present invention, it is preferable that the method has the following features.

The air velocity Vf in the fiber bundle passage in a direction approximately parallel to the running direction of the acrylic fiber bundle and the air velocity V in the heat treatment chamber in a direction approximately parallel to the running direction of the acrylic fiber bundle satisfy the conditions (3) and (4).
(3) 1.5m/s ≦ Vf ≦ 10m/s
(4) 1.5m/s ≦ V ≦ 6m/s
On the supply surface, the wind speed Vn in the direction perpendicular to the running direction of the acrylic fiber bundle satisfies the condition (5).

(5) 0.1m/s ≦ Vn ≦ 5m/s
The temperature of the hot air supplied from the supply surface is 210°C or higher and 295°C or lower.

The single fiber fineness of the acrylic fiber bundle before heat treatment is 0.05 to 0.22 tex.

Furthermore, the method for producing a carbon fiber bundle of the present invention has the following configuration.
The method for producing a carbon fiber bundle includes pre-carbonizing the flame-resistant fiber bundle obtained by the above-mentioned method for producing a flame-resistant fiber bundle in an inert atmosphere at a maximum temperature of 300 to 1,000°C to obtain a pre-carbonized fiber bundle, and then carbonizing the pre-carbonized fiber bundle in an inert atmosphere at a maximum temperature of 1,000 to 2,000°C.

Here, the "direction substantially parallel to the running direction of the acrylic fiber bundle" in the present invention means a direction within a range of ±0.7° based on the horizontal line between the apexes of a pair of opposing turn-back rollers arranged at both ends on the outside of the furnace body.

Here, the "fiber bundle passage" of the present invention refers to a space around the acrylic fiber bundle formed along the running direction of the acrylic fiber bundle, and refers to a space between two supply nozzles adjacent in the vertical direction, or a space between a supply nozzle and the top surface of the furnace body, or a space between a supply nozzle and the bottom surface of the furnace body.

Furthermore, the flame-proofing furnace of the present invention has the following configuration.
A flame-proofing furnace for heat-treating an acrylic fiber bundle, comprising:
(i) a furnace body having a slit through which the aligned fiber bundle can enter and exit;
(ii) a plurality of supply nozzles arranged at one end of the heat treatment chamber in the running direction of the fiber bundle and spaced apart from each other in the vertical direction to supply hot air into the furnace body;
(iii) a plurality of exhaust nozzles arranged at a distance from each other in the vertical direction at the other end of the furnace body in the traveling direction of the fiber bundle, and exhausting the hot air supplied from the supply nozzle from the heat treatment chamber; and (iv) at least one blower device that circulates the hot air through the supply nozzle and the exhaust nozzle.
(v) at least one heating device disposed on a flow path of the circulating hot air;
(vi) a guide roller disposed at both ends outside the furnace body, passing between adjacent supply nozzles and between adjacent discharge nozzles, and causing the fiber bundle to run in the heat treatment chamber while turning back and forth multiple times,
(vii) the supply nozzle has, on an upper surface and/or a lower surface, a supply surface for supplying a first hot air stream to a fiber bundle passing passage located above and/or below the supply nozzle, and an auxiliary supply surface for supplying a second hot air stream to a side surface of the supply nozzle facing the heat treatment chamber,
(viii) A flame-proofing furnace, comprising an adjusting means for adjusting the wind speed of the first hot air and the wind speed of the second hot air supplied from the supply nozzle.

According to the method for producing a flame-resistant fiber bundle of the present invention, the heating performance and heat removal performance of the acrylic fiber bundle passing through the furnace body of a flame-resistant furnace are improved, so that high-quality flame-resistant fiber bundles and carbon fiber bundles can be efficiently produced.

FIG. 2 is a schematic cross-sectional view of a flame-resistant furnace used in an embodiment of the present invention. FIG. 2 is a partially enlarged cross-sectional view showing the vicinity of a supply nozzle and the vicinity of a discharge nozzle used in an embodiment of the present invention. FIG. 4 is a schematic diagram showing an airflow pattern from the periphery of a supply nozzle to the periphery of a discharge nozzle used in an embodiment of the present invention. FIG. 11 is a partially enlarged cross-sectional view showing the vicinity of a supply nozzle and the vicinity of a discharge nozzle used in another embodiment of the present invention. FIG. 13 is a schematic diagram showing an airflow pattern from the vicinity of a supply nozzle to the vicinity of a discharge nozzle in the related art.

Hereinafter, an embodiment of the present invention will be described in detail with reference to Figs. 1 to 4. Fig. 1 is a schematic cross-sectional view of a flame-resistant furnace used in an embodiment of the present invention, and Fig. 2 is a partially enlarged cross-sectional view from the periphery of the supply nozzle to the periphery of the exhaust nozzle in the flame-resistant furnace used in another embodiment of the present invention. Fig. 3 is a schematic diagram showing an airflow pattern from the periphery of the supply nozzle to the periphery of the exhaust nozzle used in an embodiment of the present invention. Note that these drawings are conceptual diagrams for accurately conveying the gist of the present invention, and are simplified, and the flame-resistant furnace used in the present invention is not particularly limited, and its dimensions can be changed according to the embodiment.

The present invention is a method for producing a flame-resistant fiber bundle by heat-treating an acrylic fiber bundle 2 in an oxidizing atmosphere, which is carried out in a flame-resistant furnace through which an oxidizing gas flows. As shown in Fig. 1, the flame-resistant furnace 1 has a heat treatment chamber 3 in which hot air is blown onto the acrylic fiber bundle 2 which repeatedly travels through multiple travel areas in the furnace body 18 by turning it back with a guide roller 4 provided outside the furnace body 18, thereby performing a flame-resistant treatment. The acrylic fiber bundle 2 is fed into the furnace body 18 through a slit 17 provided in a side wall of the furnace body 18, travels in a substantially straight line inside the heat treatment chamber 3, and is once sent out of the furnace body 18 through a slit 17 in the opposite side wall. Thereafter, it is turned back by the guide rollers 4 provided on both sides outside the furnace body 18, and is fed into the furnace body 18 again. In this way, the acrylic fiber bundle 2 is turned back and forth in the running direction multiple times by the multiple guide rollers 4, and is repeatedly fed into and out of the heat treatment chamber 3 multiple times, moving in multiple stages within the heat treatment chamber 3 from top to bottom in Fig. 1 as a whole. The moving direction may be from bottom to top, and the number of times the acrylic fiber bundle 2 is turned back and forth within the heat treatment chamber 3 is not particularly limited and is appropriately designed depending on the scale of the flame-resistant furnace 1, etc. In Fig. 1, the guide rollers 4 are provided outside the furnace body 18, but the guide rollers 4 may be provided inside the furnace body 18.

While traveling in the heat treatment chamber 3 while being folded back, the acrylic fiber bundle 2 is heated by hot air flowing from the supply nozzle 5 toward the discharge surface 7 of the discharge nozzle 14, and the flame-resistant treatment progresses, resulting in a flame-resistant fiber bundle. As described above, this flame-resistant furnace 1 is a parallel-flow type ETE hot air type flame-resistant furnace. The acrylic fiber bundle 2 has a wide sheet-like form in which a plurality of fibers are aligned in parallel in a direction perpendicular to the paper surface.

The oxidizing gas flowing in the heat treatment chamber 3 may be air or the like, which is heated to a desired temperature by a heater 8 before entering the heat treatment chamber 3, and the amount of air is controlled by a blower 9, and then blown into the heat treatment chamber 3 from the supply surface 6 and/or auxiliary supply surface 12 of the supply nozzle 5. Here, the supply surface 6 of the supply nozzle 5 is a supply surface provided opposite the supply nozzle 5 adjacent to the upper and lower surfaces of the supply nozzle 5, and here, the auxiliary supply surface 12 of the supply nozzle 5 is a supply surface provided on the side of the supply nozzle 5 facing the discharge nozzle 14. The oxidizing gas discharged outside the heat treatment chamber 3 from the discharge surface 7 of the discharge nozzle 14 is released into the atmosphere after unnecessary substances are treated in an exhaust gas treatment furnace (not shown), but it is not necessary that all of the oxidizing gas is treated, and a part of the oxidizing gas may be blown again into the heat treatment chamber 3 from the supply nozzle 5 through a circulation path without being treated. In the following description, the supply surface 6 of the supply nozzle 5 will be abbreviated as simply the supply surface 6 , the auxiliary supply surface 12 of the supply nozzle 5 as simply the auxiliary supply surface 12 , and the discharge surface 7 of the discharge nozzle 14 as simply the discharge surface 7 .

The heater 8 used in the flame-resistant furnace 1 is not particularly limited as long as it has a desired heating function, and may be a known heater such as an electric heater. The blower 9 is also not particularly limited as long as it has a desired blowing function, and may be a known blower such as an axial flow fan.

In addition, the guide rollers 4 can control the running speed and tension of the acrylic fiber bundle 2 by changing the rotation speed of each roller, and this can be determined depending on the required physical properties of the flame-resistant fiber bundle and the processing amount per unit time.

Furthermore, by carving grooves at a predetermined interval and number into the surface layer of the guide roller 4, or by arranging comb guides (not shown) at a predetermined interval and number immediately adjacent to the guide roller 4, it is possible to control the intervals and number of acrylic fiber bundles 2 running in parallel.

Conventionally, it has been known that in order to improve productivity, it is sufficient to increase the running speed of the acrylic fiber bundle 2 or to increase the number of acrylic fiber bundles per unit distance in the width direction of the flame-resistant furnace 1, i.e., the yarn density. However, when such conditions for improving productivity are applied, the supply amount of the acrylic fiber bundle 2 brought into the furnace body 18 increases relative to the supply amount of hot air to the furnace body 18 per unit time, so that the heat amount of the hot air that can be used to heat or remove heat from the acrylic fiber bundle 2 decreases relatively. As a result, the temperature controllability of the acrylic fiber bundle 2 decreases, and the quality is likely to deteriorate. Therefore, it is possible to increase the amount of heat used to heat or remove heat from the acrylic fiber bundle 2 by providing another heating or heat removal means or by increasing the supply amount of hot air, but there is a problem that this leads to a significant increase in costs such as an increase in equipment costs and utility costs.

To solve this problem, it is effective to improve the heat transfer efficiency between the acrylic fiber bundle 2 and the hot air, and examples of the method include increasing the wind speed of the hot air or making the angle between the running direction of the acrylic fiber bundle 2 and the wind direction of the hot air perpendicular to each other. Another example of the method is to improve the heat transfer efficiency by spreading the fibers constituting the acrylic fiber bundle 2 to increase the surface area. However, spreading the fibers makes it easier for the acrylic fiber bundles 2 running adjacent to each other to become entangled. As described above, increasing the wind speed of the hot air increases the utility costs and causes an increase in running costs, and changing the wind direction of the hot air to a direction perpendicular to the acrylic fiber bundle 2 increases the swaying of the acrylic fiber bundle 2, making it easier for the acrylic fiber bundles 2 running adjacent to each other to become entangled. Furthermore, when the length of the flame-resistant furnace is increased in order to enlarge the flame-resistant furnace of the ETE thermal method, which has low equipment costs, the acrylic fiber bundle 2 generates excessive heat in the latter half of one pass, making it impossible to control the temperature of the acrylic fiber bundle 2, and other various disadvantages are involved in improving productivity.

The method for producing a flame-retardant fiber bundle of the present invention is a method for efficiently producing a high-quality flame-retardant fiber bundle, which is the result of extensive research into the above-mentioned problems. That is, the inventors have found that it is possible to improve the heat transfer efficiency between the acrylic fiber bundle 2 and the hot air while suppressing increases in equipment costs and running costs and suppressing entanglement of the acrylic fiber bundles 2. Hereinafter, the principle of improving the heat transfer efficiency between the acrylic fiber bundle 2 traveling in the heat treatment chamber 3 and the hot air, which is the most important point of the present invention, will be described in detail.

First, in order to clarify the difference between the conventional technology and the present invention, the airflow pattern in the furnace body 18 constructed by the conventional technology will be described with reference to Fig. 5. The length of the airflow arrow in Fig. 5 indicates the magnitude of the wind speed.

In Fig. 5, the hot air supplied from the first supply surface 19 of the supply nozzle 5 installed at one end in the furnace body 18 passes through the fiber bundle passing passage 10 between the supply nozzles 5, and when it reaches the joining surface 13 where the fiber bundle passing passage 10 and the heat treatment chamber 3 come into contact, it joins with the hot air supplied from the second supply surface 20 and flows through the heat treatment chamber 3 while gradually reducing the speed difference between the two. In the conventional technology, the speed of the airflow in the fiber bundle direction in the fiber bundle passing passage 10 originating from the hot air supplied from the first supply surface 19 was slower than the speed of the airflow originating from the hot air supplied from the second supply surface 20. For this reason, the air speed in the vicinity of the acrylic fiber bundle 2 immediately after passing the joining surface 13 is maintained in the heat treatment chamber 3 at the speed of the airflow flowing in from the fiber bundle passing passage 10, but is gradually accelerated by the joining of the airflow originating from the hot air supplied from the second supply surface 20. The merged airflow then reaches the exhaust nozzle 14 installed at the other end of the furnace body 18, most of it is exhausted from the exhaust surface 7, and a portion of it passes between the exhaust nozzles 14 and flows out of the furnace body 18.

Here, the temperature of the acrylic fiber bundle 2 when the thread density of the acrylic fiber bundle 2 is made large (or when the running speed of the acrylic fiber bundle 2 is made fast) in order to improve productivity will be described. The acrylic fiber bundle 2 that has gone out of the furnace body 18 is once cooled by the outside air and then re-enters the fiber bundle passing passage 10 to be heated again. However, when the thread density of the acrylic fiber bundle 2 is made large, the amount of heat required for heat transfer increases, making it difficult for the acrylic fiber bundle 2 to be heated/removed, and the temperature cannot be sufficiently increased in the heat treatment chamber 3. In particular, when the wind speed Vf of the fiber bundle passing passage 10 is small, the acrylic fiber bundle 2 with insufficient temperature increase enters the heat treatment chamber 3 as it is, causing the temperature of the heat treatment chamber 3 to decrease, making it increasingly difficult to increase the temperature of the acrylic fiber bundle 2. As described above, the air flow velocity in the vicinity of the acrylic fiber bundle 2 has the greatest effect on heat transfer, and therefore the acrylic fiber bundle 2 on the supply nozzle 5 side of the heat treatment chamber 3 is significantly affected by the flow velocity Vf of the hot air passing through the fiber bundle passing passage 10.

In contrast, in the airflow configuration of the embodiment of the present invention, as shown in FIG. 3 , hot air is supplied from a supply surface 6 of a supply nozzle 5 provided above and/or below the acrylic fiber bundle 2, and the air speed Vf in the fiber bundle passing flow path 10 in a direction approximately parallel to the running direction of the acrylic fiber bundle 2 and the air speed V in the heat treatment chamber 3 in a direction approximately parallel to the running direction of the acrylic fiber bundle 2 are set so as to satisfy the conditions (1) and (2).
(1) 1.5m/s ≦ Vf ≦ 15m/s
(2) 1.5m/s≦V≦10m/s.

Here, the wind speed Vf in the fiber bundle passing passage 10 in a direction approximately parallel to the running direction of the acrylic fiber bundle 2 is the average value of measured values at three points in the width direction, including the center in the width direction of the furnace body 3, on a line where the joining surface 13 and the acrylic fiber bundle 2 intersect, and the wind speed V in the direction approximately parallel to the running direction of the acrylic fiber bundle 2 in the heat treatment chamber 3 is the average value of measured values at three points in the width direction, including the center in the width direction of the furnace body 3, on a line where the cross section at the center in the running direction of the acrylic fiber bundle 2 in the heat treatment chamber 3 intersects with the acrylic fiber bundle 2. Here, the measured values at each of the three points in the width direction, including the center in the width direction of the furnace body 3, are the average value of 30 measured values taken every second using a thermal anemometer. In the above, the line where the joining surface 13 and the acrylic fiber bundle 2 intersect refers to the line where the joining surface 13 intersects with an imaginary plane including a plurality of acrylic fiber bundles 2 running parallel to the machine width direction, and the line where the acrylic fiber bundle 2 intersects with the cross section at the center of the running direction of the acrylic fiber bundle 2 in the heat treatment chamber 3 refers to the line where the cross section at the center of the running direction of the acrylic fiber bundle 2 in the heat treatment chamber 3 intersects with an imaginary plane including a plurality of acrylic fiber bundles 2 running parallel to the machine width direction. Therefore, the measurement points are included in the imaginary plane including a plurality of acrylic fiber bundles 2 running parallel to the machine width direction, but since Vf and V are indexes that respectively indicate the wind speeds in the fiber bundle passing passage 10 and in the vicinity of the acrylic fiber bundle 2 in the heat treatment chamber 3, the arrows indicating Vf and V are shown in the vicinity of the acrylic fiber bundle 2 rather than overlapping it.

When these conditions are satisfied, the acrylic fiber bundle 2 is hit by hot air having a high speed from the supply surface 6 in contact with the fiber bundle passage 10, and the heat transfer between the acrylic fiber bundle 2 and the hot air is greatly promoted. Then, this hot air changes direction parallel to the running direction of the acrylic fiber bundle 2 and flows near the acrylic fiber bundle 2 in the fiber bundle passage 10, and the heat transfer is further promoted and the acrylic fiber bundle 2 is rapidly heated. Furthermore, the hot air also flows near the acrylic fiber bundle 2 while maintaining its speed for the time being in the heat treatment chamber 3, so that the heat transfer between the acrylic fiber bundle 2 and the hot air is promoted and the temperature of the acrylic fiber bundle 2 can be controlled with high accuracy. Therefore, even when the air speed V passing through the heat treatment chamber 3 is reduced, the temperature of the acrylic fiber bundle 2 can be controlled, and therefore the amount of hot air circulating in the flame-resistant furnace 1 can be reduced. In addition, by supplying the hot air colliding with the acrylic fiber bundle 2 only near the supply nozzle 5 close to the guide roller 4, that is, locally at a position where the hanging amount of the acrylic fiber bundle 2 is relatively small, it is possible to improve heat transfer without significantly increasing the shaking of the acrylic fiber bundle 2.

Furthermore, the hot air that has passed through the fiber bundle passing passage 10 flows while expanding in the vertical direction when it reaches the heat treatment chamber 3. At this time, by supplying some hot air from the auxiliary supply surface 12 as well, it is possible to suppress turbulence of the air flow caused by the expansion of the hot air, and in turn, it is possible to suppress fiber mixing caused by the shaking of the acrylic fiber bundle 2.

Here, a method for adjusting the wind speed of the hot air supplied from the supply surface 6 and the auxiliary supply surface 12 may be appropriately provided with adjustment means such as installing an adjustment valve such as a damper in the circulation flow path leading to each supply surface, or arranging straightening members such as perforated plates or honeycombs with different opening ratios on each supply surface.

As described above, in the flame-proofing method of the present invention, it is extremely important that, in the fiber bundle passing passage 10, high-speed hot air is supplied in a direction perpendicular to the acrylic fiber bundle 2, and that the wind speed Vf in the fiber bundle passing passage 10 in a direction substantially parallel to the running direction of the acrylic fiber bundle 2 and the wind speed V in the heat treatment chamber in a direction substantially parallel to the running direction of the acrylic fiber bundle 2 satisfy the above conditions (1) and (2), which were not taken into consideration in the prior art. More preferably, the wind speed Vf in the fiber bundle passing passage 10 in a direction substantially parallel to the running direction of the acrylic fiber bundle 2 and the wind speed V in the heat treatment chamber 3 in a direction substantially parallel to the running direction of the acrylic fiber bundle 2 satisfy the conditions (3) and (4), thereby maximizing the effects of the present invention.
(3) 1.5m/s ≦ Vf ≦ 10m/s
(4) 1.5m/s≦V≦6m/s.

When the conditions (1) and (2) are not satisfied, if the wind speed Vf is less than 1.5 m/s, it may not be possible to sufficiently heat/remove heat from the acrylic fiber bundle 2. If the wind speed Vf is more than 15 m/s, the resistance that the acrylic fiber bundle 2 receives from the hot air increases, which may increase yarn sway.

If the wind speed V is less than 1.5 m/s, the acrylic fiber bundle 2 may not be sufficiently heated/removed in the heat treatment chamber 3. If the wind speed V is more than 10 m/s, the resistance of the hot air to the acrylic fiber bundle 2 may increase, resulting in increased yarn sway. Furthermore, if the wind speed V is more than 10 m/s, the amount of hot air circulating in the flame-resistant furnace may become excessive, resulting in increased utility costs.

Furthermore, it is more preferable that the wind speed Vn in the direction perpendicular to the running direction of the acrylic fiber bundle 2 on the supply surface 6 satisfies the condition (5). This makes it possible to significantly improve the heat removal and heating of the acrylic fiber bundle 2 while suppressing at a high level the yarn sway caused by the resistance of the hot air that the acrylic fiber bundle 2 receives. Here, when the wind speed Vn is less than 0.1 m/s, the heat transfer of the acrylic fiber bundle 2 is not sufficiently obtained, and the temperature may not be increased. When the wind speed Vn exceeds 5 m/s, the yarn sway may increase. Furthermore, it is preferable to set the wind speed Vn to 3.5 m/s or less, so that the effect of the present invention can be maximized.

(5) 0.1m/s≦Vn≦5m/s.
Here, the wind speed Vn in the direction perpendicular to the acrylic fiber bundle 2 on the supply surface 6 is the average value of the measured values at three points in the width direction including the center of the width direction of the furnace body 3 in the direction perpendicular to the fiber bundle running direction on the supply surface 6. Note that the measured values at three points in the width direction including the center of the width direction of the furnace body 3 in the above description are each the average value of 30 measured values every second.

Moreover, the effect of improving the heat transfer performance becomes more remarkable by setting the temperature of the hot air supplied from the supply surface 6 to be 210° C. or more and 295° C. or less. In this case, the temperatures of the hot air supplied from the supply surface 6 and the auxiliary supply surface 12 may be different, but from the viewpoints of temperature controllability of the acrylic fiber bundle 2 and equipment costs, it is preferable that they are the same.

Next, another embodiment of the present invention will be described with reference to Fig. 4. The installation position of the supply surface 6 of the supply nozzle 5 is not limited to both sides of the supply nozzle 5, and may be only the lower surface (not shown) or only the upper surface (not shown). By arranging the supply surface 6 only on the upper surface, the acrylic fiber bundle 2 can be suppressed in the direction of gravity, so that the effect of reducing yarn sway can be expected. Furthermore, by arranging the supply surface 6 on both sides, when the wind speed Vf passing through the fiber bundle passing passage 10 is constant, the supplied wind speed can be halved, so that the turbulence of the air flow around the acrylic fiber bundle 2 can be reduced, which is preferable because yarn sway can be further reduced.

The location of the supply surface 6 of the supply nozzle 5 is not limited to being located near the outside of the furnace body (FIG. 2), but may be located near the inside of the furnace body, may be arranged in multiple separate locations, or may be installed over the entire surface (FIG. 4).

Furthermore, the auxiliary supply surface 12 of the supply nozzle 5 may not be provided, and hot air may be supplied only from the supply surface 6. In this case, since there is no auxiliary supply surface 12, in order to avoid airflow turbulence caused by a sudden widening of the flow path from the fiber bundle passing flow path 10 in the heat treatment chamber 3, a straightening plate 16 may be provided to divide the heat treatment chamber 3 only around the running position of the acrylic fiber bundle 2, thereby minimizing it (FIG. 4).

In addition, by changing the angle between the main flow direction of the hot air supplied from the supply surface 6 and the running direction of the acrylic fiber bundle, various effects can be achieved. For example, by making the angle other than perpendicular, it is possible to suppress the turbulence of the hot air caused by the collision between the acrylic fiber bundle and the supply nozzle 5. In addition, by inclining the main flow direction of the hot air toward the heat treatment chamber 3, it becomes easier for part of the hot air to head toward the heat treatment chamber 3, and it is possible to suppress leakage to the outside of the flame-proofing furnace 1. In addition, by making the main flow direction of the hot air perpendicular to the running direction of the acrylic fiber bundle, it is possible to improve the heat transfer efficiency of the acrylic fiber bundle 2. In this way, the main flow direction of the hot air can be determined according to the performance required for the acrylic fiber bundle 2 and the flame-proofing furnace.

Furthermore, it is preferable that the volume of hot air sucked in from the discharge surface 7 is greater than the sum of the volumes of hot air supplied from the supply surface 6 of the supply nozzle 5 and the auxiliary supply surface 12. This makes it easier for the hot air supplied from the supply surface 6 to flow into the heat treatment chamber 3, suppressing leakage of hot air from the heat treatment chamber 3 and improving sealing performance.

It is also preferable that the throughput of the acrylic fiber bundle 2 per meter of the width of the flame-resistant furnace is 0.14 to 11 kg/min. The greater the throughput, the more remarkable the effect of improving heat transfer.

Furthermore, in the method for producing a flame-resistant fiber bundle of the present invention, the single fiber fineness of the acrylic fiber bundle 2 is preferably 0.05 to 0.22 tex, more preferably 0.05 to 0.17 tex. By setting it in this preferred range, the single fibers are less likely to be entangled when adjacent acrylic fiber bundles 2 come into contact with each other, effectively preventing intermixing of the acrylic fiber bundles, while allowing heat to be sufficiently distributed to the inner layer of the single fiber in the furnace body of the flame-resistant furnace, making it difficult for the acrylic fiber bundle 2 to fluff, and effectively preventing large intermixing of fibers, so that the quality and operability of the flame-resistant fiber bundle are more superior. Thus, the larger the single fiber fineness, the more the effect of the present invention, which is high heat transfer efficiency, is expressed, and it becomes possible to sufficiently distribute heat to the inner layer of the single fiber.

The flame-resistant fiber bundle produced by the above-mentioned method is preferably produced by subjecting a pre-carbonization treatment in an inert atmosphere at a maximum temperature of 300 to 1,000°C to produce a pre-carbonized fiber bundle, and then subjecting the pre-carbonized fiber bundle to a carbonization treatment in an inert atmosphere at a maximum temperature of 1,000 to 2,000°C to produce a carbon fiber bundle.

The maximum temperature of the inert atmosphere in the pre-carbonization treatment is more preferably 550 to 800° C. As the inert atmosphere filling the pre-carbonization furnace, a known inert atmosphere such as nitrogen, argon, or helium can be used, with nitrogen being preferred from the standpoint of economy.

The precarbonized fiber obtained by the precarbonization treatment is then fed into a carbonization furnace and carbonized. In order to improve the mechanical properties of the carbon fiber, it is more preferable to perform the carbonization treatment in an inert atmosphere at a maximum temperature of 1,200 to 2,000°C.

The inert atmosphere filling the carbonization furnace can be a known inert atmosphere such as nitrogen, argon, helium, etc., but nitrogen is preferable from the viewpoint of economy.

The carbon fiber bundle thus obtained may be provided with a sizing agent in order to improve the handling property and affinity with the matrix resin. The type of sizing agent is not particularly limited as long as the desired properties can be obtained, and examples thereof include sizing agents mainly composed of epoxy resin, polyether resin, epoxy-modified polyurethane resin, and polyester resin. The sizing agent can be provided by a known method.

Furthermore, the carbon fiber bundles may be subjected to electrolytic oxidation treatment or oxidation treatment, if necessary, for the purpose of improving the affinity and adhesiveness with the matrix resin of the fiber reinforced composite material.

The acrylic fiber bundle used as the fiber bundle to be heat-treated in the method for producing a flame-resistant fiber bundle of the present invention is preferably made of an acrylic fiber containing 100 mol % of acrylonitrile or an acrylic copolymer fiber containing 90 mol % or more of acrylonitrile. As the copolymerization component in the acrylic copolymer fiber, acrylic acid, methacrylic acid, itaconic acid, and alkali metal salts thereof, ammonium metal salts thereof, acrylamide, methyl acrylate, etc. are preferred, but the chemical properties, physical properties, dimensions, etc. of the acrylic fiber bundle are not particularly limited.

The present invention will be described in more detail below with reference to the drawings using examples, but the present invention is not limited thereto. In each example and comparative example, the wind speed and the amount of yarn sway were measured by the following method.

(1) Method for Measuring Single Fiber Fineness of Acrylic Fiber Bundle The acrylic fiber bundle was sampled before being fed into a flame-resistant furnace, and the measurement was performed in accordance with JIS L 1013 (revised version 2010-06-21).

(2) Method of measuring wind speed: An Anemomaster high temperature anemometer Model 6162 manufactured by Nippon Kanomax Co., Ltd. was used as a thermal anemometer, and the average of 30 instantaneous wind speed measurements taken every second was used. A measurement probe was inserted from a measurement hole (not shown) on the side of the furnace body 18, and the average of three measurements taken in the width direction including the center in the width direction on the line where the joining surface 13 and the acrylic fiber bundle 2 intersect was measured as Vf, the average of three measurements taken in the width direction including the center in the width direction on the line where the acrylic fiber bundle 2 intersects with the central cross section in the running direction of the acrylic fiber bundle 2 in the heat treatment chamber 3 was measured as V, and the average of three measurements taken in the width direction including the center in the width direction on the supply surface 6 in the direction perpendicular to the running direction of the acrylic fiber bundle 2 was measured as Vn.

(3) Method for Measuring the Temperature of the Acrylic Fiber Bundle A K-type thermocouple was fastened to the traveling acrylic fiber bundle 2, and the temperature of the acrylic fiber bundle 2 in the heat treatment chamber 3 was measured every second to calculate the yarn temperature uniformity rate I (%).
I(n) = (time from when the temperature of the thermocouple reaches T°C or higher to when it falls below T-5°C/time passing through the heat treatment chamber) x 100 (%)
Here, T is the temperature T of the hot air supplied from the supply nozzle 5, and I is the arithmetic mean value of the values obtained by measuring I(n) five times.

(4) Operability and quality The criteria for judgment were as follows:

(Operational feasibility)
The evaluation was based on the frequency of troubles such as fiber mixing and fiber bundle breakage per day during 10 days of continuous operation.
Excellent: Average of zero times (extremely good level)
Good: Average of 1-9 times (sufficient level for continuous operation)
Passable: Average of 10 to 19 times (level at which continuous operation can be barely continued)
Unacceptable: Average of 20 times or more (level at which continuous operation cannot be continued).

(quality)
The flame-resistant fiber bundle after the flame-resistant treatment step was visually observed over 10 m, and the number of fluffs of 10 mm or more that could be seen per 1 m on the flame-resistant fiber bundle was used for the evaluation.
Excellent: Average of 1 or less (The fluff quality has no effect on the processability or high-level processability of the product)
Good: More than 1 on average but less than 10 on average (fuzz quality has almost no effect on processability or high-level processability as a product)
Acceptable: 10 or more and less than 20 on average (fuzz quality often affects processability and high-level processability of the product)
Unacceptable: 20 or more on average (fuzz quality at a level that adversely affects passability in the process and high-level processability as a product).

[Example 1]
The heat treatment furnace of the present invention shown in Fig. 1 is used as a flame-resistant furnace for producing carbon fibers. At one end of the furnace body 18, a plurality of supply nozzles 5 serving as a supply source of hot air are installed above and below the acrylic fiber bundle 2 traveling in the furnace body 18. As shown in Fig. 2, supply surfaces 6 are provided on both the top and bottom of the supply nozzle 5, and auxiliary supply surfaces 12 are provided in the traveling direction of the acrylic fiber bundle 2. In addition, perforated plates with an opening ratio of 30% are provided on the supply surface 6 and the auxiliary supply surface 12 so that the wind speed in the width direction is uniform, and dampers (not shown) are provided in the circulation flow paths leading to each supply surface to adjust the wind speed of the hot air supplied from each supply surface.

As for the acrylic fiber bundle 2 running in the furnace body, 100 acrylic fiber bundles 2 each consisting of 20,000 single fibers with a single fiber fineness of 0.11 dtex were aligned and heat-treated in a flame-resistant furnace 1 to obtain a flame-resistant fiber bundle. The horizontal distance (roll span) L' between the guide rollers 4 at both ends outside the furnace body 18 of the flame-resistant furnace 1 was 15 m, the guide rollers 4 were grooved rollers, and the pitch interval (groove pitch) Wp was 10 mm. The temperature of the oxidizing gas in the heat treatment chamber 3 of the flame-resistant furnace 1 at this time was 240 to 280°C. The running speed of the acrylic fiber bundle 2 was adjusted in the range of 1 to 15 m/min in accordance with the flame-resistant furnace length L so that a sufficient flame-resistant treatment time was obtained, and the process tension was adjusted in the range of 0.5 to 2.5 g/dtex.

The obtained flame-retardant fiber bundle was then baked at a maximum temperature of 700° C. in a pre-carbonization furnace, and then baked at a maximum temperature of 1,400° C. in a carbonization furnace. After electrolytic surface treatment, a sizing agent was applied to obtain a carbon fiber bundle.

As shown in Table 1, the yarn temperature uniformity was 20% when the wind speed Vn at the supply surface 6 was 8.5 m/s, the wind speed Vf at the fiber bundle passage 10 was 11.2 m/s, and the average wind speed V in the heat treatment chamber 3 was 7.0 m/s. Under the above conditions, during the flame-retardant treatment of the acrylic fiber bundle 2, there was little intermixing or fiber bundle breakage due to contact between the acrylic fiber bundles, and a flame-retardant fiber bundle was obtained with good operability. In addition, visual inspection of the obtained flame-retardant fiber bundle revealed that it was of good quality with little fluff.

[Example 2]
The wind speed Vn on the supply surface was 6.0 m/s, the wind speed Vf in the fiber bundle passage 10 was 3.3 m/s, and the average wind speed V in the heat treatment chamber 3 was 3.0 m/s, and the rest were the same as in Example 1. At this time, the yarn temperature uniformity rate was 17%. Under the above conditions, during the flame-resistant treatment of the acrylic fiber bundle 2, no intermixing or fiber bundle breakage due to contact between the acrylic fiber bundles occurred at all, and a flame-resistant fiber bundle was obtained with extremely good operability. In addition, visual inspection of the obtained flame-resistant fiber bundle revealed that it was of good quality with little fluff.

[Example 3]
The wind speed Vn on the supply surface was 3.3 m/s, and the rest was the same as in Example 2. At this time, the yarn temperature uniformity rate was 16%. Under the above conditions, during the flame-retardant treatment of the acrylic fiber bundle 2, no intermixing or fiber bundle breakage due to contact between the acrylic fiber bundles occurred, and a flame-retardant fiber bundle was obtained with extremely good operability. In addition, as a result of visually inspecting the obtained flame-retardant fiber bundle, it was of extremely good quality with no fluff, etc.

[Comparative Example 1]
In Comparative Example 1, the wind speed Vf in the fiber passage 10 was 1.1 m/s, and the average wind speed V in the heat treatment chamber 3 was 6.0 m/s, and the rest were the same as in Example 2. In this case, the yarn temperature uniformity rate was 8%, and under the above conditions, during the flame-retardant treatment of the acrylic fiber bundle 2, there were many cases of mixed fibers due to contact between the acrylic fiber bundles and single fiber breakage. In addition, visual inspection of the obtained flame-retardant fiber bundle revealed a lot of fluff and poor quality.

The present invention relates to a method for producing a flame-retardant fiber bundle and a method for producing a carbon fiber bundle, and can be applied to aircraft applications, industrial applications such as pressure vessels and windmills, and sports applications such as golf shafts, but the range of applications is not limited to these.

Reference Signs List 1: Flame-resistant furnace 2: Acrylic fiber bundle 3: Heat treatment chamber 4: Guide roller 5: Supply nozzle 6: Supply surface 7: Discharge surface 8: Heater 9: Blower 10: Fiber bundle passage 12: Auxiliary supply surface 13: Junction surface 14: Discharge nozzle 16: Straightening plate 17: Slit 18: Furnace body 19: First supply surface 20: Second supply surface

Claims (7)

A method for producing a flame-resistant fiber bundle, comprising heat-treating an aligned acrylic fiber bundle in an oxidizing atmosphere while folding back the aligned acrylic fiber bundle with guide rollers disposed on both ends outside a furnace body of a hot air heating type flame-resistant furnace, wherein hot air is supplied from a supply surface disposed above and/or below the acrylic fiber bundle in a fiber bundle passage above and/or below a supply nozzle disposed at one end in the running direction of the acrylic fiber bundle for supplying hot air into a heat treatment chamber, the supply nozzle being disposed at one end in the running direction of the acrylic fiber bundle, and the wind speed Vf in the fiber bundle passage in a direction approximately parallel to the running direction of the acrylic fiber bundle and the wind speed V in the heat treatment chamber in a direction approximately parallel to the running direction of the acrylic fiber bundle satisfy conditions (1) and (2).
(1) 1.5m/s ≦ Vf ≦ 15m/s
(2) 1.5m/s ≦ V ≦ 10m/s 2. The method for producing a flame-resistant fiber bundle according to claim 1, wherein an air velocity Vf in the fiber bundle passage in a direction approximately parallel to the running direction of the acrylic fiber bundle and an air velocity V in the heat treatment chamber in a direction approximately parallel to the running direction of the acrylic fiber bundle satisfy conditions (3) and (4).
(3) 1.5m/s ≦ Vf ≦ 10m/s
(4) 1.5m/s ≦ V ≦ 6m/s 3. The method for producing a flame-resistant fiber bundle according to claim 1, wherein a wind speed Vn in a direction perpendicular to a running direction of the acrylic fiber bundle on the supply surface satisfies condition (5).
(5) 0.1m/s ≦ Vn ≦ 5m/s The method for producing a flame-resistant fiber bundle according to any one of claims 1 to 3, wherein the temperature of the hot air supplied from the supply surface is 210°C or higher and 295°C or lower. The method for producing a flame-resistant fiber bundle according to any one of claims 1 to 4, wherein the single fiber fineness of the acrylic fiber bundle before heat treatment is 0.05 to 0.22 tex. A method for producing a carbon fiber bundle, comprising: pre-carbonizing a flame-resistant fiber bundle obtained by the method for producing a flame-resistant fiber bundle according to any one of claims 1 to 5 at a maximum temperature of 300 to 1,000°C in an inert atmosphere to obtain a pre-carbonized fiber bundle; and then carbonizing the pre-carbonized fiber bundle at a maximum temperature of 1,000 to 2,000°C in an inert atmosphere. A flame-proofing furnace for heat-treating an acrylic fiber bundle, comprising:
(i) a furnace body having a slit through which the aligned fiber bundle can enter and exit;
(ii ) a plurality of supply nozzles arranged at one end of the heat treatment chamber in the running direction of the fiber bundle and spaced apart from each other in the vertical direction, for supplying hot air into the furnace body;
(iii) a plurality of exhaust nozzles arranged at a distance from each other in the vertical direction at the other end of the furnace body in the traveling direction of the fiber bundle, and exhausting the hot air supplied from the supply nozzles from the heat treatment chamber;
(iv) at least one blower for circulating hot air through the supply nozzle and the exhaust nozzle;
(v) at least one heating device disposed on a flow path of the circulating hot air;
(vi) a guide roller disposed at both ends outside the furnace body, passing between adjacent supply nozzles and between adjacent discharge nozzles, and causing the fiber bundle to run in the heat treatment chamber while turning back and forth multiple times,
(vii) the supply nozzle has, on an upper surface and/or a lower surface, a supply surface for supplying a first hot air stream to a fiber bundle passing passage located above and/or below the supply nozzle, and an auxiliary supply surface for supplying a second hot air stream to a side surface of the supply nozzle facing the heat treatment chamber,
(viii) A flame-proofing furnace comprising an adjusting means for adjusting the wind speed of the first hot air and the wind speed of the second hot air supplied from the supply nozzle.

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